API

This part of the documentation covers all the interfaces of Flask. For parts where Flask depends on external libraries, we document the most important right here and provide links to the canonical documentation.

Application Object

class flask.Flask(import_name, static_url_path=None, static_folder='static', static_host=None, host_matching=False, subdomain_matching=False, template_folder='templates', instance_path=None, instance_relative_config=False, root_path=None)

The flask object implements a WSGI application and acts as the central object. It is passed the name of the module or package of the application. Once it is created it will act as a central registry for the view functions, the URL rules, template configuration and much more.

The name of the package is used to resolve resources from inside the package or the folder the module is contained in depending on if the package parameter resolves to an actual python package (a folder with an __init__.py file inside) or a standard module (just a .py file).

For more information about resource loading, see open_resource().

Usually you create a Flask instance in your main module or in the __init__.py file of your package like this:

from flask import Flask
app = Flask(__name__)

About the First Parameter

The idea of the first parameter is to give Flask an idea of what belongs to your application. This name is used to find resources on the filesystem, can be used by extensions to improve debugging information and a lot more.

So it’s important what you provide there. If you are using a single module, __name__ is always the correct value. If you however are using a package, it’s usually recommended to hardcode the name of your package there.

For example if your application is defined in yourapplication/app.py you should create it with one of the two versions below:

app = Flask('yourapplication')
app = Flask(__name__.split('.')[0])

Why is that? The application will work even with __name__, thanks to how resources are looked up. However it will make debugging more painful. Certain extensions can make assumptions based on the import name of your application. For example the Flask-SQLAlchemy extension will look for the code in your application that triggered an SQL query in debug mode. If the import name is not properly set up, that debugging information is lost. (For example it would only pick up SQL queries in yourapplication.app and not yourapplication.views.frontend)

Changelog

New in version 1.0: The host_matching and static_host parameters were added.

New in version 1.0: The subdomain_matching parameter was added. Subdomain matching needs to be enabled manually now. Setting SERVER_NAME does not implicitly enable it.

New in version 0.11: The root_path parameter was added.

New in version 0.8: The instance_path and instance_relative_config parameters were added.

New in version 0.7: The static_url_path, static_folder, and template_folder parameters were added.

Parameters:
  • import_name – the name of the application package
  • static_url_path – can be used to specify a different path for the static files on the web. Defaults to the name of the static_folder folder.
  • static_folder – the folder with static files that should be served at static_url_path. Defaults to the 'static' folder in the root path of the application.
  • static_host – the host to use when adding the static route. Defaults to None. Required when using host_matching=True with a static_folder configured.
  • host_matching – set url_map.host_matching attribute. Defaults to False.
  • subdomain_matching – consider the subdomain relative to SERVER_NAME when matching routes. Defaults to False.
  • template_folder – the folder that contains the templates that should be used by the application. Defaults to 'templates' folder in the root path of the application.
  • instance_path – An alternative instance path for the application. By default the folder 'instance' next to the package or module is assumed to be the instance path.
  • instance_relative_config – if set to True relative filenames for loading the config are assumed to be relative to the instance path instead of the application root.
  • root_path – Flask by default will automatically calculate the path to the root of the application. In certain situations this cannot be achieved (for instance if the package is a Python 3 namespace package) and needs to be manually defined.
add_template_filter(f, name=None)

Register a custom template filter. Works exactly like the template_filter() decorator.

Parameters:name – the optional name of the filter, otherwise the function name will be used.
add_template_global(f, name=None)

Register a custom template global function. Works exactly like the template_global() decorator.

Changelog

New in version 0.10.

Parameters:name – the optional name of the global function, otherwise the function name will be used.
add_template_test(f, name=None)

Register a custom template test. Works exactly like the template_test() decorator.

Changelog

New in version 0.10.

Parameters:name – the optional name of the test, otherwise the function name will be used.
add_url_rule(rule, endpoint=None, view_func=None, provide_automatic_options=None, **options)

Connects a URL rule. Works exactly like the route() decorator. If a view_func is provided it will be registered with the endpoint.

Basically this example:

@app.route('/')
def index():
    pass

Is equivalent to the following:

def index():
    pass
app.add_url_rule('/', 'index', index)

If the view_func is not provided you will need to connect the endpoint to a view function like so:

app.view_functions['index'] = index

Internally route() invokes add_url_rule() so if you want to customize the behavior via subclassing you only need to change this method.

For more information refer to URL Route Registrations.

Changelog

Changed in version 0.6: OPTIONS is added automatically as method.

Changed in version 0.2: view_func parameter added.

Parameters:
  • rule – the URL rule as string
  • endpoint – the endpoint for the registered URL rule. Flask itself assumes the name of the view function as endpoint
  • view_func – the function to call when serving a request to the provided endpoint
  • provide_automatic_options – controls whether the OPTIONS method should be added automatically. This can also be controlled by setting the view_func.provide_automatic_options = False before adding the rule.
  • options – the options to be forwarded to the underlying Rule object. A change to Werkzeug is handling of method options. methods is a list of methods this rule should be limited to (GET, POST etc.). By default a rule just listens for GET (and implicitly HEAD). Starting with Flask 0.6, OPTIONS is implicitly added and handled by the standard request handling.
after_request(f)

Register a function to be run after each request.

Your function must take one parameter, an instance of response_class and return a new response object or the same (see process_response()).

As of Flask 0.7 this function might not be executed at the end of the request in case an unhandled exception occurred.

after_request_funcs = None

A dictionary with lists of functions that should be called after each request. The key of the dictionary is the name of the blueprint this function is active for, None for all requests. This can for example be used to close database connections. To register a function here, use the after_request() decorator.

app_context()

Create an AppContext. Use as a with block to push the context, which will make current_app point at this application.

An application context is automatically pushed by RequestContext.push() when handling a request, and when running a CLI command. Use this to manually create a context outside of these situations.

with app.app_context():
    init_db()

See The Application Context.

Changelog

New in version 0.9.

app_ctx_globals_class

alias of flask.ctx._AppCtxGlobals

auto_find_instance_path()

Tries to locate the instance path if it was not provided to the constructor of the application class. It will basically calculate the path to a folder named instance next to your main file or the package.

Changelog

New in version 0.8.

before_first_request(f)

Registers a function to be run before the first request to this instance of the application.

The function will be called without any arguments and its return value is ignored.

Changelog

New in version 0.8.

before_first_request_funcs = None

A list of functions that will be called at the beginning of the first request to this instance. To register a function, use the before_first_request() decorator.

Changelog

New in version 0.8.

before_request(f)

Registers a function to run before each request.

For example, this can be used to open a database connection, or to load the logged in user from the session.

The function will be called without any arguments. If it returns a non-None value, the value is handled as if it was the return value from the view, and further request handling is stopped.

before_request_funcs = None

A dictionary with lists of functions that will be called at the beginning of each request. The key of the dictionary is the name of the blueprint this function is active for, or None for all requests. To register a function, use the before_request() decorator.

blueprints = None

all the attached blueprints in a dictionary by name. Blueprints can be attached multiple times so this dictionary does not tell you how often they got attached.

Changelog

New in version 0.7.

cli = None

The click command line context for this application. Commands registered here show up in the flask command once the application has been discovered. The default commands are provided by Flask itself and can be overridden.

This is an instance of a click.Group object.

config = None

The configuration dictionary as Config. This behaves exactly like a regular dictionary but supports additional methods to load a config from files.

config_class

alias of flask.config.Config

context_processor(f)

Registers a template context processor function.

create_global_jinja_loader()

Creates the loader for the Jinja2 environment. Can be used to override just the loader and keeping the rest unchanged. It’s discouraged to override this function. Instead one should override the jinja_loader() function instead.

The global loader dispatches between the loaders of the application and the individual blueprints.

Changelog

New in version 0.7.

create_jinja_environment()

Creates the Jinja2 environment based on jinja_options and select_jinja_autoescape(). Since 0.7 this also adds the Jinja2 globals and filters after initialization. Override this function to customize the behavior.

Changelog

Changed in version 0.11: Environment.auto_reload set in accordance with TEMPLATES_AUTO_RELOAD configuration option.

New in version 0.5.

create_url_adapter(request)

Creates a URL adapter for the given request. The URL adapter is created at a point where the request context is not yet set up so the request is passed explicitly.

Changelog

Changed in version 1.0: SERVER_NAME no longer implicitly enables subdomain matching. Use subdomain_matching instead.

Changed in version 0.9: This can now also be called without a request object when the URL adapter is created for the application context.

New in version 0.6.

debug

Whether debug mode is enabled. When using flask run to start the development server, an interactive debugger will be shown for unhandled exceptions, and the server will be reloaded when code changes. This maps to the DEBUG config key. This is enabled when env is 'development' and is overridden by the FLASK_DEBUG environment variable. It may not behave as expected if set in code.

Do not enable debug mode when deploying in production.

Default: True if env is 'development', or False otherwise.

default_config = {'APPLICATION_ROOT': '/', 'DEBUG': None, 'ENV': None, 'EXPLAIN_TEMPLATE_LOADING': False, 'JSONIFY_MIMETYPE': 'application/json', 'JSONIFY_PRETTYPRINT_REGULAR': False, 'JSON_AS_ASCII': True, 'JSON_SORT_KEYS': True, 'MAX_CONTENT_LENGTH': None, 'MAX_COOKIE_SIZE': 4093, 'PERMANENT_SESSION_LIFETIME': datetime.timedelta(31), 'PREFERRED_URL_SCHEME': 'http', 'PRESERVE_CONTEXT_ON_EXCEPTION': None, 'PROPAGATE_EXCEPTIONS': None, 'SECRET_KEY': None, 'SEND_FILE_MAX_AGE_DEFAULT': datetime.timedelta(0, 43200), 'SERVER_NAME': None, 'SESSION_COOKIE_DOMAIN': None, 'SESSION_COOKIE_HTTPONLY': True, 'SESSION_COOKIE_NAME': 'session', 'SESSION_COOKIE_PATH': None, 'SESSION_COOKIE_SAMESITE': None, 'SESSION_COOKIE_SECURE': False, 'SESSION_REFRESH_EACH_REQUEST': True, 'TEMPLATES_AUTO_RELOAD': None, 'TESTING': False, 'TRAP_BAD_REQUEST_ERRORS': None, 'TRAP_HTTP_EXCEPTIONS': False, 'USE_X_SENDFILE': False}

Default configuration parameters.

dispatch_request()

Does the request dispatching. Matches the URL and returns the return value of the view or error handler. This does not have to be a response object. In order to convert the return value to a proper response object, call make_response().

Changelog

Changed in version 0.7: This no longer does the exception handling, this code was moved to the new full_dispatch_request().

do_teardown_appcontext(exc=<object object>)

Called right before the application context is popped.

When handling a request, the application context is popped after the request context. See do_teardown_request().

This calls all functions decorated with teardown_appcontext(). Then the appcontext_tearing_down signal is sent.

This is called by AppContext.pop().

Changelog

New in version 0.9.

do_teardown_request(exc=<object object>)

Called after the request is dispatched and the response is returned, right before the request context is popped.

This calls all functions decorated with teardown_request(), and Blueprint.teardown_request() if a blueprint handled the request. Finally, the request_tearing_down signal is sent.

This is called by RequestContext.pop(), which may be delayed during testing to maintain access to resources.

Parameters:exc – An unhandled exception raised while dispatching the request. Detected from the current exception information if not passed. Passed to each teardown function.
Changelog

Changed in version 0.9: Added the exc argument.

endpoint(endpoint)

A decorator to register a function as an endpoint. Example:

@app.endpoint('example.endpoint')
def example():
    return "example"
Parameters:endpoint – the name of the endpoint
env

What environment the app is running in. Flask and extensions may enable behaviors based on the environment, such as enabling debug mode. This maps to the ENV config key. This is set by the FLASK_ENV environment variable and may not behave as expected if set in code.

Do not enable development when deploying in production.

Default: 'production'

error_handler_spec = None

A dictionary of all registered error handlers. The key is None for error handlers active on the application, otherwise the key is the name of the blueprint. Each key points to another dictionary where the key is the status code of the http exception. The special key None points to a list of tuples where the first item is the class for the instance check and the second the error handler function.

To register an error handler, use the errorhandler() decorator.

errorhandler(code_or_exception)

Register a function to handle errors by code or exception class.

A decorator that is used to register a function given an error code. Example:

@app.errorhandler(404)
def page_not_found(error):
    return 'This page does not exist', 404

You can also register handlers for arbitrary exceptions:

@app.errorhandler(DatabaseError)
def special_exception_handler(error):
    return 'Database connection failed', 500
Changelog

New in version 0.7: Use register_error_handler() instead of modifying error_handler_spec directly, for application wide error handlers.

New in version 0.7: One can now additionally also register custom exception types that do not necessarily have to be a subclass of the HTTPException class.

Parameters:code_or_exception – the code as integer for the handler, or an arbitrary exception
extensions = None

a place where extensions can store application specific state. For example this is where an extension could store database engines and similar things. For backwards compatibility extensions should register themselves like this:

if not hasattr(app, 'extensions'):
    app.extensions = {}
app.extensions['extensionname'] = SomeObject()

The key must match the name of the extension module. For example in case of a “Flask-Foo” extension in flask_foo, the key would be 'foo'.

Changelog

New in version 0.7.

full_dispatch_request()

Dispatches the request and on top of that performs request pre and postprocessing as well as HTTP exception catching and error handling.

Changelog

New in version 0.7.

get_send_file_max_age(filename)

Provides default cache_timeout for the send_file() functions.

By default, this function returns SEND_FILE_MAX_AGE_DEFAULT from the configuration of current_app.

Static file functions such as send_from_directory() use this function, and send_file() calls this function on current_app when the given cache_timeout is None. If a cache_timeout is given in send_file(), that timeout is used; otherwise, this method is called.

This allows subclasses to change the behavior when sending files based on the filename. For example, to set the cache timeout for .js files to 60 seconds:

class MyFlask(flask.Flask):
    def get_send_file_max_age(self, name):
        if name.lower().endswith('.js'):
            return 60
        return flask.Flask.get_send_file_max_age(self, name)
Changelog

New in version 0.9.

got_first_request

This attribute is set to True if the application started handling the first request.

Changelog

New in version 0.8.

handle_exception(e)

Default exception handling that kicks in when an exception occurs that is not caught. In debug mode the exception will be re-raised immediately, otherwise it is logged and the handler for a 500 internal server error is used. If no such handler exists, a default 500 internal server error message is displayed.

Changelog

New in version 0.3.

handle_http_exception(e)

Handles an HTTP exception. By default this will invoke the registered error handlers and fall back to returning the exception as response.

Changelog

New in version 0.3.

handle_url_build_error(error, endpoint, values)

Handle BuildError on url_for().

handle_user_exception(e)

This method is called whenever an exception occurs that should be handled. A special case are HTTPExceptions which are forwarded by this function to the handle_http_exception() method. This function will either return a response value or reraise the exception with the same traceback.

Changelog

Changed in version 1.0: Key errors raised from request data like form show the the bad key in debug mode rather than a generic bad request message.

New in version 0.7.

has_static_folder

This is True if the package bound object’s container has a folder for static files.

Changelog

New in version 0.5.

import_name = None

The name of the package or module that this app belongs to. Do not change this once it is set by the constructor.

inject_url_defaults(endpoint, values)

Injects the URL defaults for the given endpoint directly into the values dictionary passed. This is used internally and automatically called on URL building.

Changelog

New in version 0.7.

instance_path = None

Holds the path to the instance folder.

Changelog

New in version 0.8.

iter_blueprints()

Iterates over all blueprints by the order they were registered.

Changelog

New in version 0.11.

jinja_env

The Jinja2 environment used to load templates.

jinja_environment

alias of flask.templating.Environment

jinja_loader

The Jinja loader for this package bound object.

Changelog

New in version 0.5.

jinja_options = {'extensions': ['jinja2.ext.autoescape', 'jinja2.ext.with_']}

Options that are passed directly to the Jinja2 environment.

json_decoder

alias of flask.json.JSONDecoder

json_encoder

alias of flask.json.JSONEncoder

log_exception(exc_info)

Logs an exception. This is called by handle_exception() if debugging is disabled and right before the handler is called. The default implementation logs the exception as error on the logger.

Changelog

New in version 0.8.

logger

The 'flask.app' logger, a standard Python Logger.

In debug mode, the logger’s level will be set to DEBUG.

If there are no handlers configured, a default handler will be added. See Logging for more information.

Changelog

Changed in version 1.0: Behavior was simplified. The logger is always named flask.app. The level is only set during configuration, it doesn’t check app.debug each time. Only one format is used, not different ones depending on app.debug. No handlers are removed, and a handler is only added if no handlers are already configured.

New in version 0.3.

make_config(instance_relative=False)

Used to create the config attribute by the Flask constructor. The instance_relative parameter is passed in from the constructor of Flask (there named instance_relative_config) and indicates if the config should be relative to the instance path or the root path of the application.

Changelog

New in version 0.8.

make_default_options_response()

This method is called to create the default OPTIONS response. This can be changed through subclassing to change the default behavior of OPTIONS responses.

Changelog

New in version 0.7.

make_null_session()

Creates a new instance of a missing session. Instead of overriding this method we recommend replacing the session_interface.

Changelog

New in version 0.7.

make_response(rv)

Convert the return value from a view function to an instance of response_class.

Parameters:rv

the return value from the view function. The view function must return a response. Returning None, or the view ending without returning, is not allowed. The following types are allowed for view_rv:

str (unicode in Python 2)
A response object is created with the string encoded to UTF-8 as the body.
bytes (str in Python 2)
A response object is created with the bytes as the body.
tuple
Either (body, status, headers), (body, status), or (body, headers), where body is any of the other types allowed here, status is a string or an integer, and headers is a dictionary or a list of (key, value) tuples. If body is a response_class instance, status overwrites the exiting value and headers are extended.
response_class
The object is returned unchanged.
other Response class
The object is coerced to response_class.
callable()
The function is called as a WSGI application. The result is used to create a response object.
Changelog

Changed in version 0.9: Previously a tuple was interpreted as the arguments for the response object.

make_shell_context()

Returns the shell context for an interactive shell for this application. This runs all the registered shell context processors.

Changelog

New in version 0.11.

name

The name of the application. This is usually the import name with the difference that it’s guessed from the run file if the import name is main. This name is used as a display name when Flask needs the name of the application. It can be set and overridden to change the value.

Changelog

New in version 0.8.

open_instance_resource(resource, mode='rb')

Opens a resource from the application’s instance folder (instance_path). Otherwise works like open_resource(). Instance resources can also be opened for writing.

Parameters:
  • resource – the name of the resource. To access resources within subfolders use forward slashes as separator.
  • mode – resource file opening mode, default is ‘rb’.
open_resource(resource, mode='rb')

Opens a resource from the application’s resource folder. To see how this works, consider the following folder structure:

/myapplication.py
/schema.sql
/static
    /style.css
/templates
    /layout.html
    /index.html

If you want to open the schema.sql file you would do the following:

with app.open_resource('schema.sql') as f:
    contents = f.read()
    do_something_with(contents)
Parameters:
  • resource – the name of the resource. To access resources within subfolders use forward slashes as separator.
  • mode – resource file opening mode, default is ‘rb’.
open_session(request)

Creates or opens a new session. Default implementation stores all session data in a signed cookie. This requires that the secret_key is set. Instead of overriding this method we recommend replacing the session_interface.

Parameters:request – an instance of request_class.
permanent_session_lifetime

A timedelta which is used to set the expiration date of a permanent session. The default is 31 days which makes a permanent session survive for roughly one month.

This attribute can also be configured from the config with the PERMANENT_SESSION_LIFETIME configuration key. Defaults to timedelta(days=31)

preprocess_request()

Called before the request is dispatched. Calls url_value_preprocessors registered with the app and the current blueprint (if any). Then calls before_request_funcs registered with the app and the blueprint.

If any before_request() handler returns a non-None value, the value is handled as if it was the return value from the view, and further request handling is stopped.

preserve_context_on_exception

Returns the value of the PRESERVE_CONTEXT_ON_EXCEPTION configuration value in case it’s set, otherwise a sensible default is returned.

Changelog

New in version 0.7.

process_response(response)

Can be overridden in order to modify the response object before it’s sent to the WSGI server. By default this will call all the after_request() decorated functions.

Changelog

Changed in version 0.5: As of Flask 0.5 the functions registered for after request execution are called in reverse order of registration.

Parameters:response – a response_class object.
Returns:a new response object or the same, has to be an instance of response_class.
propagate_exceptions

Returns the value of the PROPAGATE_EXCEPTIONS configuration value in case it’s set, otherwise a sensible default is returned.

Changelog

New in version 0.7.

register_blueprint(blueprint, **options)

Register a Blueprint on the application. Keyword arguments passed to this method will override the defaults set on the blueprint.

Calls the blueprint’s register() method after recording the blueprint in the application’s blueprints.

Parameters:
  • blueprint – The blueprint to register.
  • url_prefix – Blueprint routes will be prefixed with this.
  • subdomain – Blueprint routes will match on this subdomain.
  • url_defaults – Blueprint routes will use these default values for view arguments.
  • options – Additional keyword arguments are passed to BlueprintSetupState. They can be accessed in record() callbacks.
Changelog

New in version 0.7.

register_error_handler(code_or_exception, f)

Alternative error attach function to the errorhandler() decorator that is more straightforward to use for non decorator usage.

Changelog

New in version 0.7.

request_class

alias of flask.wrappers.Request

request_context(environ)

Create a RequestContext representing a WSGI environment. Use a with block to push the context, which will make request point at this request.

See The Request Context.

Typically you should not call this from your own code. A request context is automatically pushed by the wsgi_app() when handling a request. Use test_request_context() to create an environment and context instead of this method.

Parameters:environ – a WSGI environment
response_class

alias of flask.wrappers.Response

root_path = None

Absolute path to the package on the filesystem. Used to look up resources contained in the package.

route(rule, **options)

A decorator that is used to register a view function for a given URL rule. This does the same thing as add_url_rule() but is intended for decorator usage:

@app.route('/')
def index():
    return 'Hello World'

For more information refer to URL Route Registrations.

Parameters:
  • rule – the URL rule as string
  • endpoint – the endpoint for the registered URL rule. Flask itself assumes the name of the view function as endpoint
  • options – the options to be forwarded to the underlying Rule object. A change to Werkzeug is handling of method options. methods is a list of methods this rule should be limited to (GET, POST etc.). By default a rule just listens for GET (and implicitly HEAD). Starting with Flask 0.6, OPTIONS is implicitly added and handled by the standard request handling.
run(host=None, port=None, debug=None, load_dotenv=True, **options)

Runs the application on a local development server.

Do not use run() in a production setting. It is not intended to meet security and performance requirements for a production server. Instead, see Deployment Options for WSGI server recommendations.

If the debug flag is set the server will automatically reload for code changes and show a debugger in case an exception happened.

If you want to run the application in debug mode, but disable the code execution on the interactive debugger, you can pass use_evalex=False as parameter. This will keep the debugger’s traceback screen active, but disable code execution.

It is not recommended to use this function for development with automatic reloading as this is badly supported. Instead you should be using the flask command line script’s run support.

Keep in Mind

Flask will suppress any server error with a generic error page unless it is in debug mode. As such to enable just the interactive debugger without the code reloading, you have to invoke run() with debug=True and use_reloader=False. Setting use_debugger to True without being in debug mode won’t catch any exceptions because there won’t be any to catch.

Parameters:
  • host – the hostname to listen on. Set this to '0.0.0.0' to have the server available externally as well. Defaults to '127.0.0.1' or the host in the SERVER_NAME config variable if present.
  • port – the port of the webserver. Defaults to 5000 or the port defined in the SERVER_NAME config variable if present.
  • debug – if given, enable or disable debug mode. See debug.
  • load_dotenv – Load the nearest .env and .flaskenv files to set environment variables. Will also change the working directory to the directory containing the first file found.
  • options – the options to be forwarded to the underlying Werkzeug server. See werkzeug.serving.run_simple() for more information.
Changelog

Changed in version 1.0: If installed, python-dotenv will be used to load environment variables from .env and .flaskenv files.

If set, the FLASK_ENV and FLASK_DEBUG environment variables will override env and debug.

Threaded mode is enabled by default.

Changed in version 0.10: The default port is now picked from the SERVER_NAME variable.

save_session(session, response)

Saves the session if it needs updates. For the default implementation, check open_session(). Instead of overriding this method we recommend replacing the session_interface.

Parameters:
secret_key

If a secret key is set, cryptographic components can use this to sign cookies and other things. Set this to a complex random value when you want to use the secure cookie for instance.

This attribute can also be configured from the config with the SECRET_KEY configuration key. Defaults to None.

select_jinja_autoescape(filename)

Returns True if autoescaping should be active for the given template name. If no template name is given, returns True.

Changelog

New in version 0.5.

send_file_max_age_default

A timedelta which is used as default cache_timeout for the send_file() functions. The default is 12 hours.

This attribute can also be configured from the config with the SEND_FILE_MAX_AGE_DEFAULT configuration key. This configuration variable can also be set with an integer value used as seconds. Defaults to timedelta(hours=12)

send_static_file(filename)

Function used internally to send static files from the static folder to the browser.

Changelog

New in version 0.5.

The secure cookie uses this for the name of the session cookie.

This attribute can also be configured from the config with the SESSION_COOKIE_NAME configuration key. Defaults to 'session'

session_interface = <flask.sessions.SecureCookieSessionInterface object>

the session interface to use. By default an instance of SecureCookieSessionInterface is used here.

Changelog

New in version 0.8.

shell_context_processor(f)

Registers a shell context processor function.

Changelog

New in version 0.11.

shell_context_processors = None

A list of shell context processor functions that should be run when a shell context is created.

Changelog

New in version 0.11.

should_ignore_error(error)

This is called to figure out if an error should be ignored or not as far as the teardown system is concerned. If this function returns True then the teardown handlers will not be passed the error.

Changelog

New in version 0.10.

static_folder

The absolute path to the configured static folder.

static_url_path

The URL prefix that the static route will be registered for.

teardown_appcontext(f)

Registers a function to be called when the application context ends. These functions are typically also called when the request context is popped.

Example:

ctx = app.app_context()
ctx.push()
...
ctx.pop()

When ctx.pop() is executed in the above example, the teardown functions are called just before the app context moves from the stack of active contexts. This becomes relevant if you are using such constructs in tests.

Since a request context typically also manages an application context it would also be called when you pop a request context.

When a teardown function was called because of an unhandled exception it will be passed an error object. If an errorhandler() is registered, it will handle the exception and the teardown will not receive it.

The return values of teardown functions are ignored.

Changelog

New in version 0.9.

teardown_appcontext_funcs = None

A list of functions that are called when the application context is destroyed. Since the application context is also torn down if the request ends this is the place to store code that disconnects from databases.

Changelog

New in version 0.9.

teardown_request(f)

Register a function to be run at the end of each request, regardless of whether there was an exception or not. These functions are executed when the request context is popped, even if not an actual request was performed.

Example:

ctx = app.test_request_context()
ctx.push()
...
ctx.pop()

When ctx.pop() is executed in the above example, the teardown functions are called just before the request context moves from the stack of active contexts. This becomes relevant if you are using such constructs in tests.

Generally teardown functions must take every necessary step to avoid that they will fail. If they do execute code that might fail they will have to surround the execution of these code by try/except statements and log occurring errors.

When a teardown function was called because of an exception it will be passed an error object.

The return values of teardown functions are ignored.

Debug Note

In debug mode Flask will not tear down a request on an exception immediately. Instead it will keep it alive so that the interactive debugger can still access it. This behavior can be controlled by the PRESERVE_CONTEXT_ON_EXCEPTION configuration variable.

teardown_request_funcs = None

A dictionary with lists of functions that are called after each request, even if an exception has occurred. The key of the dictionary is the name of the blueprint this function is active for, None for all requests. These functions are not allowed to modify the request, and their return values are ignored. If an exception occurred while processing the request, it gets passed to each teardown_request function. To register a function here, use the teardown_request() decorator.

Changelog

New in version 0.7.

template_context_processors = None

A dictionary with list of functions that are called without argument to populate the template context. The key of the dictionary is the name of the blueprint this function is active for, None for all requests. Each returns a dictionary that the template context is updated with. To register a function here, use the context_processor() decorator.

template_filter(name=None)

A decorator that is used to register custom template filter. You can specify a name for the filter, otherwise the function name will be used. Example:

@app.template_filter()
def reverse(s):
    return s[::-1]
Parameters:name – the optional name of the filter, otherwise the function name will be used.
template_folder = None

Location of the template files to be added to the template lookup. None if templates should not be added.

template_global(name=None)

A decorator that is used to register a custom template global function. You can specify a name for the global function, otherwise the function name will be used. Example:

@app.template_global()
def double(n):
    return 2 * n
Changelog

New in version 0.10.

Parameters:name – the optional name of the global function, otherwise the function name will be used.
template_test(name=None)

A decorator that is used to register custom template test. You can specify a name for the test, otherwise the function name will be used. Example:

@app.template_test()
def is_prime(n):
    if n == 2:
        return True
    for i in range(2, int(math.ceil(math.sqrt(n))) + 1):
        if n % i == 0:
            return False
    return True
Changelog

New in version 0.10.

Parameters:name – the optional name of the test, otherwise the function name will be used.
templates_auto_reload

Reload templates when they are changed. Used by create_jinja_environment().

This attribute can be configured with TEMPLATES_AUTO_RELOAD. If not set, it will be enabled in debug mode.

Changelog

New in version 1.0: This property was added but the underlying config and behavior already existed.

test_cli_runner(**kwargs)

Create a CLI runner for testing CLI commands. See Testing CLI Commands.

Returns an instance of test_cli_runner_class, by default FlaskCliRunner. The Flask app object is passed as the first argument.

Changelog

New in version 1.0.

test_cli_runner_class = None

The CliRunner subclass, by default FlaskCliRunner that is used by test_cli_runner(). Its __init__ method should take a Flask app object as the first argument.

Changelog

New in version 1.0.

test_client(use_cookies=True, **kwargs)

Creates a test client for this application. For information about unit testing head over to Testing Flask Applications.

Note that if you are testing for assertions or exceptions in your application code, you must set app.testing = True in order for the exceptions to propagate to the test client. Otherwise, the exception will be handled by the application (not visible to the test client) and the only indication of an AssertionError or other exception will be a 500 status code response to the test client. See the testing attribute. For example:

app.testing = True
client = app.test_client()

The test client can be used in a with block to defer the closing down of the context until the end of the with block. This is useful if you want to access the context locals for testing:

with app.test_client() as c:
    rv = c.get('/?vodka=42')
    assert request.args['vodka'] == '42'

Additionally, you may pass optional keyword arguments that will then be passed to the application’s test_client_class constructor. For example:

from flask.testing import FlaskClient

class CustomClient(FlaskClient):
    def __init__(self, *args, **kwargs):
        self._authentication = kwargs.pop("authentication")
        super(CustomClient,self).__init__( *args, **kwargs)

app.test_client_class = CustomClient
client = app.test_client(authentication='Basic ....')

See FlaskClient for more information.

Changelog

Changed in version 0.11: Added **kwargs to support passing additional keyword arguments to the constructor of test_client_class.

New in version 0.7: The use_cookies parameter was added as well as the ability to override the client to be used by setting the test_client_class attribute.

Changed in version 0.4: added support for with block usage for the client.

test_client_class = None

the test client that is used with when test_client is used.

Changelog

New in version 0.7.

test_request_context(*args, **kwargs)

Create a RequestContext for a WSGI environment created from the given values. This is mostly useful during testing, where you may want to run a function that uses request data without dispatching a full request.

See The Request Context.

Use a with block to push the context, which will make request point at the request for the created environment.

with test_request_context(...):
    generate_report()

When using the shell, it may be easier to push and pop the context manually to avoid indentation.

ctx = app.test_request_context(...)
ctx.push()
...
ctx.pop()

Takes the same arguments as Werkzeug’s EnvironBuilder, with some defaults from the application. See the linked Werkzeug docs for most of the available arguments. Flask-specific behavior is listed here.

Parameters:
  • path – URL path being requested.
  • base_url – Base URL where the app is being served, which path is relative to. If not given, built from PREFERRED_URL_SCHEME, subdomain, SERVER_NAME, and APPLICATION_ROOT.
  • subdomain – Subdomain name to append to SERVER_NAME.
  • url_scheme – Scheme to use instead of PREFERRED_URL_SCHEME.
  • data – The request body, either as a string or a dict of form keys and values.
  • json – If given, this is serialized as JSON and passed as data. Also defaults content_type to application/json.
  • args – other positional arguments passed to EnvironBuilder.
  • kwargs – other keyword arguments passed to EnvironBuilder.
testing

The testing flag. Set this to True to enable the test mode of Flask extensions (and in the future probably also Flask itself). For example this might activate test helpers that have an additional runtime cost which should not be enabled by default.

If this is enabled and PROPAGATE_EXCEPTIONS is not changed from the default it’s implicitly enabled.

This attribute can also be configured from the config with the TESTING configuration key. Defaults to False.

trap_http_exception(e)

Checks if an HTTP exception should be trapped or not. By default this will return False for all exceptions except for a bad request key error if TRAP_BAD_REQUEST_ERRORS is set to True. It also returns True if TRAP_HTTP_EXCEPTIONS is set to True.

This is called for all HTTP exceptions raised by a view function. If it returns True for any exception the error handler for this exception is not called and it shows up as regular exception in the traceback. This is helpful for debugging implicitly raised HTTP exceptions.

Changelog

Changed in version 1.0: Bad request errors are not trapped by default in debug mode.

New in version 0.8.

update_template_context(context)

Update the template context with some commonly used variables. This injects request, session, config and g into the template context as well as everything template context processors want to inject. Note that the as of Flask 0.6, the original values in the context will not be overridden if a context processor decides to return a value with the same key.

Parameters:context – the context as a dictionary that is updated in place to add extra variables.
url_build_error_handlers = None

A list of functions that are called when url_for() raises a BuildError. Each function registered here is called with error, endpoint and values. If a function returns None or raises a BuildError the next function is tried.

Changelog

New in version 0.9.

url_default_functions = None

A dictionary with lists of functions that can be used as URL value preprocessors. The key None here is used for application wide callbacks, otherwise the key is the name of the blueprint. Each of these functions has the chance to modify the dictionary of URL values before they are used as the keyword arguments of the view function. For each function registered this one should also provide a url_defaults() function that adds the parameters automatically again that were removed that way.

Changelog

New in version 0.7.

url_defaults(f)

Callback function for URL defaults for all view functions of the application. It’s called with the endpoint and values and should update the values passed in place.

url_map = None

The Map for this instance. You can use this to change the routing converters after the class was created but before any routes are connected. Example:

from werkzeug.routing import BaseConverter

class ListConverter(BaseConverter):
    def to_python(self, value):
        return value.split(',')
    def to_url(self, values):
        return ','.join(super(ListConverter, self).to_url(value)
                        for value in values)

app = Flask(__name__)
app.url_map.converters['list'] = ListConverter
url_rule_class

alias of werkzeug.routing.Rule

url_value_preprocessor(f)

Register a URL value preprocessor function for all view functions in the application. These functions will be called before the before_request() functions.

The function can modify the values captured from the matched url before they are passed to the view. For example, this can be used to pop a common language code value and place it in g rather than pass it to every view.

The function is passed the endpoint name and values dict. The return value is ignored.

url_value_preprocessors = None

A dictionary with lists of functions that are called before the before_request_funcs functions. The key of the dictionary is the name of the blueprint this function is active for, or None for all requests. To register a function, use url_value_preprocessor().

Changelog

New in version 0.7.

use_x_sendfile

Enable this if you want to use the X-Sendfile feature. Keep in mind that the server has to support this. This only affects files sent with the send_file() method.

Changelog

New in version 0.2.

This attribute can also be configured from the config with the USE_X_SENDFILE configuration key. Defaults to False.

view_functions = None

A dictionary of all view functions registered. The keys will be function names which are also used to generate URLs and the values are the function objects themselves. To register a view function, use the route() decorator.

wsgi_app(environ, start_response)

The actual WSGI application. This is not implemented in __call__() so that middlewares can be applied without losing a reference to the app object. Instead of doing this:

app = MyMiddleware(app)

It’s a better idea to do this instead:

app.wsgi_app = MyMiddleware(app.wsgi_app)

Then you still have the original application object around and can continue to call methods on it.

Changelog

Changed in version 0.7: Teardown events for the request and app contexts are called even if an unhandled error occurs. Other events may not be called depending on when an error occurs during dispatch. See Callbacks and Errors.

Parameters:
  • environ – A WSGI environment.
  • start_response – A callable accepting a status code, a list of headers, and an optional exception context to start the response.

Blueprint Objects

class flask.Blueprint(name, import_name, static_folder=None, static_url_path=None, template_folder=None, url_prefix=None, subdomain=None, url_defaults=None, root_path=None)

Represents a blueprint. A blueprint is an object that records functions that will be called with the BlueprintSetupState later to register functions or other things on the main application. See Modular Applications with Blueprints for more information.

Changelog

New in version 0.7.

add_app_template_filter(f, name=None)

Register a custom template filter, available application wide. Like Flask.add_template_filter() but for a blueprint. Works exactly like the app_template_filter() decorator.

Parameters:name – the optional name of the filter, otherwise the function name will be used.
add_app_template_global(f, name=None)

Register a custom template global, available application wide. Like Flask.add_template_global() but for a blueprint. Works exactly like the app_template_global() decorator.

Changelog

New in version 0.10.

Parameters:name – the optional name of the global, otherwise the function name will be used.
add_app_template_test(f, name=None)

Register a custom template test, available application wide. Like Flask.add_template_test() but for a blueprint. Works exactly like the app_template_test() decorator.

Changelog

New in version 0.10.

Parameters:name – the optional name of the test, otherwise the function name will be used.
add_url_rule(rule, endpoint=None, view_func=None, **options)

Like Flask.add_url_rule() but for a blueprint. The endpoint for the url_for() function is prefixed with the name of the blueprint.

after_app_request(f)

Like Flask.after_request() but for a blueprint. Such a function is executed after each request, even if outside of the blueprint.

after_request(f)

Like Flask.after_request() but for a blueprint. This function is only executed after each request that is handled by a function of that blueprint.

app_context_processor(f)

Like Flask.context_processor() but for a blueprint. Such a function is executed each request, even if outside of the blueprint.

app_errorhandler(code)

Like Flask.errorhandler() but for a blueprint. This handler is used for all requests, even if outside of the blueprint.

app_template_filter(name=None)

Register a custom template filter, available application wide. Like Flask.template_filter() but for a blueprint.

Parameters:name – the optional name of the filter, otherwise the function name will be used.
app_template_global(name=None)

Register a custom template global, available application wide. Like Flask.template_global() but for a blueprint.

Changelog

New in version 0.10.

Parameters:name – the optional name of the global, otherwise the function name will be used.
app_template_test(name=None)

Register a custom template test, available application wide. Like Flask.template_test() but for a blueprint.

Changelog

New in version 0.10.

Parameters:name – the optional name of the test, otherwise the function name will be used.
app_url_defaults(f)

Same as url_defaults() but application wide.

app_url_value_preprocessor(f)

Same as url_value_preprocessor() but application wide.

before_app_first_request(f)

Like Flask.before_first_request(). Such a function is executed before the first request to the application.

before_app_request(f)

Like Flask.before_request(). Such a function is executed before each request, even if outside of a blueprint.

before_request(f)

Like Flask.before_request() but for a blueprint. This function is only executed before each request that is handled by a function of that blueprint.

context_processor(f)

Like Flask.context_processor() but for a blueprint. This function is only executed for requests handled by a blueprint.

endpoint(endpoint)

Like Flask.endpoint() but for a blueprint. This does not prefix the endpoint with the blueprint name, this has to be done explicitly by the user of this method. If the endpoint is prefixed with a . it will be registered to the current blueprint, otherwise it’s an application independent endpoint.

errorhandler(code_or_exception)

Registers an error handler that becomes active for this blueprint only. Please be aware that routing does not happen local to a blueprint so an error handler for 404 usually is not handled by a blueprint unless it is caused inside a view function. Another special case is the 500 internal server error which is always looked up from the application.

Otherwise works as the errorhandler() decorator of the Flask object.

get_send_file_max_age(filename)

Provides default cache_timeout for the send_file() functions.

By default, this function returns SEND_FILE_MAX_AGE_DEFAULT from the configuration of current_app.

Static file functions such as send_from_directory() use this function, and send_file() calls this function on current_app when the given cache_timeout is None. If a cache_timeout is given in send_file(), that timeout is used; otherwise, this method is called.

This allows subclasses to change the behavior when sending files based on the filename. For example, to set the cache timeout for .js files to 60 seconds:

class MyFlask(flask.Flask):
    def get_send_file_max_age(self, name):
        if name.lower().endswith('.js'):
            return 60
        return flask.Flask.get_send_file_max_age(self, name)
Changelog

New in version 0.9.

has_static_folder

This is True if the package bound object’s container has a folder for static files.

Changelog

New in version 0.5.

import_name = None

The name of the package or module that this app belongs to. Do not change this once it is set by the constructor.

jinja_loader

The Jinja loader for this package bound object.

Changelog

New in version 0.5.

json_decoder = None

Blueprint local JSON decoder class to use. Set to None to use the app’s json_decoder.

json_encoder = None

Blueprint local JSON decoder class to use. Set to None to use the app’s json_encoder.

make_setup_state(app, options, first_registration=False)

Creates an instance of BlueprintSetupState() object that is later passed to the register callback functions. Subclasses can override this to return a subclass of the setup state.

open_resource(resource, mode='rb')

Opens a resource from the application’s resource folder. To see how this works, consider the following folder structure:

/myapplication.py
/schema.sql
/static
    /style.css
/templates
    /layout.html
    /index.html

If you want to open the schema.sql file you would do the following:

with app.open_resource('schema.sql') as f:
    contents = f.read()
    do_something_with(contents)
Parameters:
  • resource – the name of the resource. To access resources within subfolders use forward slashes as separator.
  • mode – resource file opening mode, default is ‘rb’.
record(func)

Registers a function that is called when the blueprint is registered on the application. This function is called with the state as argument as returned by the make_setup_state() method.

record_once(func)

Works like record() but wraps the function in another function that will ensure the function is only called once. If the blueprint is registered a second time on the application, the function passed is not called.

register(app, options, first_registration=False)

Called by Flask.register_blueprint() to register all views and callbacks registered on the blueprint with the application. Creates a BlueprintSetupState and calls each record() callback with it.

Parameters:
  • app – The application this blueprint is being registered with.
  • options – Keyword arguments forwarded from register_blueprint().
  • first_registration – Whether this is the first time this blueprint has been registered on the application.
register_error_handler(code_or_exception, f)

Non-decorator version of the errorhandler() error attach function, akin to the register_error_handler() application-wide function of the Flask object but for error handlers limited to this blueprint.

Changelog

New in version 0.11.

root_path = None

Absolute path to the package on the filesystem. Used to look up resources contained in the package.

route(rule, **options)

Like Flask.route() but for a blueprint. The endpoint for the url_for() function is prefixed with the name of the blueprint.

send_static_file(filename)

Function used internally to send static files from the static folder to the browser.

Changelog

New in version 0.5.

static_folder

The absolute path to the configured static folder.

static_url_path

The URL prefix that the static route will be registered for.

teardown_app_request(f)

Like Flask.teardown_request() but for a blueprint. Such a function is executed when tearing down each request, even if outside of the blueprint.

teardown_request(f)

Like Flask.teardown_request() but for a blueprint. This function is only executed when tearing down requests handled by a function of that blueprint. Teardown request functions are executed when the request context is popped, even when no actual request was performed.

template_folder = None

Location of the template files to be added to the template lookup. None if templates should not be added.

url_defaults(f)

Callback function for URL defaults for this blueprint. It’s called with the endpoint and values and should update the values passed in place.

url_value_preprocessor(f)

Registers a function as URL value preprocessor for this blueprint. It’s called before the view functions are called and can modify the url values provided.

Incoming Request Data

class flask.Request(environ, populate_request=True, shallow=False)

The request object used by default in Flask. Remembers the matched endpoint and view arguments.

It is what ends up as request. If you want to replace the request object used you can subclass this and set request_class to your subclass.

The request object is a Request subclass and provides all of the attributes Werkzeug defines plus a few Flask specific ones.

environ

The underlying WSGI environment.

path
full_path
script_root
url
base_url
url_root

Provides different ways to look at the current IRI. Imagine your application is listening on the following application root:

http://www.example.com/myapplication

And a user requests the following URI:

http://www.example.com/myapplication/%CF%80/page.html?x=y

In this case the values of the above mentioned attributes would be the following:

path u'/π/page.html'
full_path u'/π/page.html?x=y'
script_root u'/myapplication'
base_url u'http://www.example.com/myapplication/π/page.html'
url u'http://www.example.com/myapplication/π/page.html?x=y'
url_root u'http://www.example.com/myapplication/'
accept_charsets

List of charsets this client supports as CharsetAccept object.

accept_encodings

List of encodings this client accepts. Encodings in a HTTP term are compression encodings such as gzip. For charsets have a look at accept_charset.

accept_languages

List of languages this client accepts as LanguageAccept object.

accept_mimetypes

List of mimetypes this client supports as MIMEAccept object.

access_route

If a forwarded header exists this is a list of all ip addresses from the client ip to the last proxy server.

classmethod application(f)

Decorate a function as responder that accepts the request as first argument. This works like the responder() decorator but the function is passed the request object as first argument and the request object will be closed automatically:

@Request.application
def my_wsgi_app(request):
    return Response('Hello World!')

As of Werkzeug 0.14 HTTP exceptions are automatically caught and converted to responses instead of failing.

Parameters:f – the WSGI callable to decorate
Returns:a new WSGI callable
args

The parsed URL parameters (the part in the URL after the question mark).

By default an ImmutableMultiDict is returned from this function. This can be changed by setting parameter_storage_class to a different type. This might be necessary if the order of the form data is important.

authorization

The Authorization object in parsed form.

base_url

Like url but without the querystring See also: trusted_hosts.

blueprint

The name of the current blueprint

cache_control

A RequestCacheControl object for the incoming cache control headers.

close()

Closes associated resources of this request object. This closes all file handles explicitly. You can also use the request object in a with statement which will automatically close it.

Changelog

New in version 0.9.

content_encoding

The Content-Encoding entity-header field is used as a modifier to the media-type. When present, its value indicates what additional content codings have been applied to the entity-body, and thus what decoding mechanisms must be applied in order to obtain the media-type referenced by the Content-Type header field.

Changelog

New in version 0.9.

content_length

The Content-Length entity-header field indicates the size of the entity-body in bytes or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

content_md5
The Content-MD5 entity-header field, as defined in RFC 1864, is an MD5 digest of the entity-body for the purpose of providing an end-to-end message integrity check (MIC) of the entity-body. (Note: a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.)
Changelog

New in version 0.9.

content_type

The Content-Type entity-header field indicates the media type of the entity-body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET.

cookies

A dict with the contents of all cookies transmitted with the request.

data

Contains the incoming request data as string in case it came with a mimetype Werkzeug does not handle.

date

The Date general-header field represents the date and time at which the message was originated, having the same semantics as orig-date in RFC 822.

dict_storage_class

alias of werkzeug.datastructures.ImmutableTypeConversionDict

endpoint

The endpoint that matched the request. This in combination with view_args can be used to reconstruct the same or a modified URL. If an exception happened when matching, this will be None.

files

MultiDict object containing all uploaded files. Each key in files is the name from the <input type="file" name="">. Each value in files is a Werkzeug FileStorage object.

It basically behaves like a standard file object you know from Python, with the difference that it also has a save() function that can store the file on the filesystem.

Note that files will only contain data if the request method was POST, PUT or PATCH and the <form> that posted to the request had enctype="multipart/form-data". It will be empty otherwise.

See the MultiDict / FileStorage documentation for more details about the used data structure.

form

The form parameters. By default an ImmutableMultiDict is returned from this function. This can be changed by setting parameter_storage_class to a different type. This might be necessary if the order of the form data is important.

Please keep in mind that file uploads will not end up here, but instead in the files attribute.

Changelog

Changed in version 0.9: Previous to Werkzeug 0.9 this would only contain form data for POST and PUT requests.

form_data_parser_class

alias of werkzeug.formparser.FormDataParser

classmethod from_values(*args, **kwargs)

Create a new request object based on the values provided. If environ is given missing values are filled from there. This method is useful for small scripts when you need to simulate a request from an URL. Do not use this method for unittesting, there is a full featured client object (Client) that allows to create multipart requests, support for cookies etc.

This accepts the same options as the EnvironBuilder.

Changelog

Changed in version 0.5: This method now accepts the same arguments as EnvironBuilder. Because of this the environ parameter is now called environ_overrides.

Returns:request object
full_path

Requested path as unicode, including the query string.

get_data(cache=True, as_text=False, parse_form_data=False)

This reads the buffered incoming data from the client into one bytestring. By default this is cached but that behavior can be changed by setting cache to False.

Usually it’s a bad idea to call this method without checking the content length first as a client could send dozens of megabytes or more to cause memory problems on the server.

Note that if the form data was already parsed this method will not return anything as form data parsing does not cache the data like this method does. To implicitly invoke form data parsing function set parse_form_data to True. When this is done the return value of this method will be an empty string if the form parser handles the data. This generally is not necessary as if the whole data is cached (which is the default) the form parser will used the cached data to parse the form data. Please be generally aware of checking the content length first in any case before calling this method to avoid exhausting server memory.

If as_text is set to True the return value will be a decoded unicode string.

Changelog

New in version 0.9.

get_json(force=False, silent=False, cache=True)

Parse and return the data as JSON. If the mimetype does not indicate JSON (application/json, see is_json()), this returns None unless force is true. If parsing fails, on_json_loading_failed() is called and its return value is used as the return value.

Parameters:
  • force – Ignore the mimetype and always try to parse JSON.
  • silent – Silence parsing errors and return None instead.
  • cache – Store the parsed JSON to return for subsequent calls.
headers

The headers from the WSGI environ as immutable EnvironHeaders.

host

Just the host including the port if available. See also: trusted_hosts.

host_url

Just the host with scheme as IRI. See also: trusted_hosts.

if_match

An object containing all the etags in the If-Match header.

Return type:ETags
if_modified_since

The parsed If-Modified-Since header as datetime object.

if_none_match

An object containing all the etags in the If-None-Match header.

Return type:ETags
if_range

The parsed If-Range header.

Changelog

New in version 0.7.

Return type:IfRange
if_unmodified_since

The parsed If-Unmodified-Since header as datetime object.

is_json

Check if the mimetype indicates JSON data, either application/json or application/*+json.

Changelog

New in version 0.11.

is_multiprocess

boolean that is True if the application is served by a WSGI server that spawns multiple processes.

is_multithread

boolean that is True if the application is served by a multithreaded WSGI server.

is_run_once

boolean that is True if the application will be executed only once in a process lifetime. This is the case for CGI for example, but it’s not guaranteed that the execution only happens one time.

is_secure

True if the request is secure.

is_xhr

True if the request was triggered via a JavaScript XMLHttpRequest. This only works with libraries that support the X-Requested-With header and set it to “XMLHttpRequest”. Libraries that do that are prototype, jQuery and Mochikit and probably some more.

Deprecated since version 0.13: X-Requested-With is not standard and is unreliable.

Changelog
json

This will contain the parsed JSON data if the mimetype indicates JSON (application/json, see is_json()), otherwise it will be None.

list_storage_class

alias of werkzeug.datastructures.ImmutableList

make_form_data_parser()

Creates the form data parser. Instantiates the form_data_parser_class with some parameters.

Changelog

New in version 0.8.

max_content_length

Read-only view of the MAX_CONTENT_LENGTH config key.

max_forwards

The Max-Forwards request-header field provides a mechanism with the TRACE and OPTIONS methods to limit the number of proxies or gateways that can forward the request to the next inbound server.

method

The request method. (For example 'GET' or 'POST').

mimetype

Like content_type, but without parameters (eg, without charset, type etc.) and always lowercase. For example if the content type is text/HTML; charset=utf-8 the mimetype would be 'text/html'.

mimetype_params

The mimetype parameters as dict. For example if the content type is text/html; charset=utf-8 the params would be {'charset': 'utf-8'}.

on_json_loading_failed(e)

Called if get_json() parsing fails and isn’t silenced. If this method returns a value, it is used as the return value for get_json(). The default implementation raises a BadRequest exception.

Changelog

Changed in version 0.10: Raise a BadRequest error instead of returning an error message as JSON. If you want that behavior you can add it by subclassing.

New in version 0.8.

parameter_storage_class

alias of werkzeug.datastructures.ImmutableMultiDict

path

Requested path as unicode. This works a bit like the regular path info in the WSGI environment but will always include a leading slash, even if the URL root is accessed.

pragma

The Pragma general-header field is used to include implementation-specific directives that might apply to any recipient along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems MAY require that behavior be consistent with the directives.

query_string

The URL parameters as raw bytestring.

range

The parsed Range header.

Changelog

New in version 0.7.

Return type:Range
referrer

The Referer[sic] request-header field allows the client to specify, for the server’s benefit, the address (URI) of the resource from which the Request-URI was obtained (the “referrer”, although the header field is misspelled).

remote_addr

The remote address of the client.

remote_user

If the server supports user authentication, and the script is protected, this attribute contains the username the user has authenticated as.

routing_exception = None

If matching the URL failed, this is the exception that will be raised / was raised as part of the request handling. This is usually a NotFound exception or something similar.

scheme

URL scheme (http or https).

Changelog

New in version 0.7.

script_root

The root path of the script without the trailing slash.

stream

If the incoming form data was not encoded with a known mimetype the data is stored unmodified in this stream for consumption. Most of the time it is a better idea to use data which will give you that data as a string. The stream only returns the data once.

Unlike input_stream this stream is properly guarded that you can’t accidentally read past the length of the input. Werkzeug will internally always refer to this stream to read data which makes it possible to wrap this object with a stream that does filtering.

Changelog

Changed in version 0.9: This stream is now always available but might be consumed by the form parser later on. Previously the stream was only set if no parsing happened.

url

The reconstructed current URL as IRI. See also: trusted_hosts.

url_charset

The charset that is assumed for URLs. Defaults to the value of charset.

Changelog

New in version 0.6.

url_root

The full URL root (with hostname), this is the application root as IRI. See also: trusted_hosts.

url_rule = None

The internal URL rule that matched the request. This can be useful to inspect which methods are allowed for the URL from a before/after handler (request.url_rule.methods) etc. Though if the request’s method was invalid for the URL rule, the valid list is available in routing_exception.valid_methods instead (an attribute of the Werkzeug exception MethodNotAllowed) because the request was never internally bound.

Changelog

New in version 0.6.

user_agent

The current user agent.

values

A werkzeug.datastructures.CombinedMultiDict that combines args and form.

view_args = None

A dict of view arguments that matched the request. If an exception happened when matching, this will be None.

want_form_data_parsed

Returns True if the request method carries content. As of Werkzeug 0.9 this will be the case if a content type is transmitted.

Changelog

New in version 0.8.

flask.request

To access incoming request data, you can use the global request object. Flask parses incoming request data for you and gives you access to it through that global object. Internally Flask makes sure that you always get the correct data for the active thread if you are in a multithreaded environment.

This is a proxy. See Notes On Proxies for more information.

The request object is an instance of a Request subclass and provides all of the attributes Werkzeug defines. This just shows a quick overview of the most important ones.

Response Objects

class flask.Response(response=None, status=None, headers=None, mimetype=None, content_type=None, direct_passthrough=False)

The response object that is used by default in Flask. Works like the response object from Werkzeug but is set to have an HTML mimetype by default. Quite often you don’t have to create this object yourself because make_response() will take care of that for you.

If you want to replace the response object used you can subclass this and set response_class to your subclass.

Changelog

Changed in version 1.0: JSON support is added to the response, like the request. This is useful when testing to get the test client response data as JSON.

Changed in version 1.0: Added max_cookie_size.

headers

A Headers object representing the response headers.

status

A string with a response status.

status_code

The response status as integer.

data

A descriptor that calls get_data() and set_data(). This should not be used and will eventually get deprecated.

get_json(force=False, silent=False, cache=True)

Parse and return the data as JSON. If the mimetype does not indicate JSON (application/json, see is_json()), this returns None unless force is true. If parsing fails, on_json_loading_failed() is called and its return value is used as the return value.

Parameters:
  • force – Ignore the mimetype and always try to parse JSON.
  • silent – Silence parsing errors and return None instead.
  • cache – Store the parsed JSON to return for subsequent calls.
is_json

Check if the mimetype indicates JSON data, either application/json or application/*+json.

Changelog

New in version 0.11.

Read-only view of the MAX_COOKIE_SIZE config key.

See max_cookie_size in Werkzeug’s docs.

mimetype

The mimetype (content type without charset etc.)

Sets a cookie. The parameters are the same as in the cookie Morsel object in the Python standard library but it accepts unicode data, too.

A warning is raised if the size of the cookie header exceeds max_cookie_size, but the header will still be set.

Parameters:
  • key – the key (name) of the cookie to be set.
  • value – the value of the cookie.
  • max_age – should be a number of seconds, or None (default) if the cookie should last only as long as the client’s browser session.
  • expires – should be a datetime object or UNIX timestamp.
  • path – limits the cookie to a given path, per default it will span the whole domain.
  • domain – if you want to set a cross-domain cookie. For example, domain=".example.com" will set a cookie that is readable by the domain www.example.com, foo.example.com etc. Otherwise, a cookie will only be readable by the domain that set it.
  • secure – If True, the cookie will only be available via HTTPS
  • httponly – disallow JavaScript to access the cookie. This is an extension to the cookie standard and probably not supported by all browsers.
  • samesite – Limits the scope of the cookie such that it will only be attached to requests if those requests are “same-site”.

Sessions

If you have set Flask.secret_key (or configured it from SECRET_KEY) you can use sessions in Flask applications. A session makes it possible to remember information from one request to another. The way Flask does this is by using a signed cookie. The user can look at the session contents, but can’t modify it unless they know the secret key, so make sure to set that to something complex and unguessable.

To access the current session you can use the session object:

class flask.session

The session object works pretty much like an ordinary dict, with the difference that it keeps track on modifications.

This is a proxy. See Notes On Proxies for more information.

The following attributes are interesting:

new

True if the session is new, False otherwise.

modified

True if the session object detected a modification. Be advised that modifications on mutable structures are not picked up automatically, in that situation you have to explicitly set the attribute to True yourself. Here an example:

# this change is not picked up because a mutable object (here
# a list) is changed.
session['objects'].append(42)
# so mark it as modified yourself
session.modified = True
permanent

If set to True the session lives for permanent_session_lifetime seconds. The default is 31 days. If set to False (which is the default) the session will be deleted when the user closes the browser.

Session Interface

Changelog

New in version 0.8.

The session interface provides a simple way to replace the session implementation that Flask is using.

class flask.sessions.SessionInterface

The basic interface you have to implement in order to replace the default session interface which uses werkzeug’s securecookie implementation. The only methods you have to implement are open_session() and save_session(), the others have useful defaults which you don’t need to change.

The session object returned by the open_session() method has to provide a dictionary like interface plus the properties and methods from the SessionMixin. We recommend just subclassing a dict and adding that mixin:

class Session(dict, SessionMixin):
    pass

If open_session() returns None Flask will call into make_null_session() to create a session that acts as replacement if the session support cannot work because some requirement is not fulfilled. The default NullSession class that is created will complain that the secret key was not set.

To replace the session interface on an application all you have to do is to assign flask.Flask.session_interface:

app = Flask(__name__)
app.session_interface = MySessionInterface()
Changelog

New in version 0.8.

Returns the domain that should be set for the session cookie.

Uses SESSION_COOKIE_DOMAIN if it is configured, otherwise falls back to detecting the domain based on SERVER_NAME.

Once detected (or if not set at all), SESSION_COOKIE_DOMAIN is updated to avoid re-running the logic.

Returns True if the session cookie should be httponly. This currently just returns the value of the SESSION_COOKIE_HTTPONLY config var.

Returns the path for which the cookie should be valid. The default implementation uses the value from the SESSION_COOKIE_PATH config var if it’s set, and falls back to APPLICATION_ROOT or uses / if it’s None.

Return 'Strict' or 'Lax' if the cookie should use the SameSite attribute. This currently just returns the value of the SESSION_COOKIE_SAMESITE setting.

Returns True if the cookie should be secure. This currently just returns the value of the SESSION_COOKIE_SECURE setting.

get_expiration_time(app, session)

A helper method that returns an expiration date for the session or None if the session is linked to the browser session. The default implementation returns now + the permanent session lifetime configured on the application.

is_null_session(obj)

Checks if a given object is a null session. Null sessions are not asked to be saved.

This checks if the object is an instance of null_session_class by default.

make_null_session(app)

Creates a null session which acts as a replacement object if the real session support could not be loaded due to a configuration error. This mainly aids the user experience because the job of the null session is to still support lookup without complaining but modifications are answered with a helpful error message of what failed.

This creates an instance of null_session_class by default.

null_session_class

make_null_session() will look here for the class that should be created when a null session is requested. Likewise the is_null_session() method will perform a typecheck against this type.

alias of NullSession

open_session(app, request)

This method has to be implemented and must either return None in case the loading failed because of a configuration error or an instance of a session object which implements a dictionary like interface + the methods and attributes on SessionMixin.

pickle_based = False

A flag that indicates if the session interface is pickle based. This can be used by Flask extensions to make a decision in regards to how to deal with the session object.

Changelog

New in version 0.10.

save_session(app, session, response)

This is called for actual sessions returned by open_session() at the end of the request. This is still called during a request context so if you absolutely need access to the request you can do that.

Used by session backends to determine if a Set-Cookie header should be set for this session cookie for this response. If the session has been modified, the cookie is set. If the session is permanent and the SESSION_REFRESH_EACH_REQUEST config is true, the cookie is always set.

This check is usually skipped if the session was deleted.

Changelog

New in version 0.11.

class flask.sessions.SecureCookieSessionInterface

The default session interface that stores sessions in signed cookies through the itsdangerous module.

static digest_method()

the hash function to use for the signature. The default is sha1

key_derivation = 'hmac'

the name of the itsdangerous supported key derivation. The default is hmac.

open_session(app, request)

This method has to be implemented and must either return None in case the loading failed because of a configuration error or an instance of a session object which implements a dictionary like interface + the methods and attributes on SessionMixin.

salt = 'cookie-session'

the salt that should be applied on top of the secret key for the signing of cookie based sessions.

save_session(app, session, response)

This is called for actual sessions returned by open_session() at the end of the request. This is still called during a request context so if you absolutely need access to the request you can do that.

serializer = <flask.json.tag.TaggedJSONSerializer object>

A python serializer for the payload. The default is a compact JSON derived serializer with support for some extra Python types such as datetime objects or tuples.

session_class

alias of SecureCookieSession

class flask.sessions.SecureCookieSession(initial=None)

Base class for sessions based on signed cookies.

This session backend will set the modified and accessed attributes. It cannot reliably track whether a session is new (vs. empty), so new remains hard coded to False.

accessed = False

header, which allows caching proxies to cache different pages for different users.

get(k[, d]) → D[k] if k in D, else d. d defaults to None.
modified = False

When data is changed, this is set to True. Only the session dictionary itself is tracked; if the session contains mutable data (for example a nested dict) then this must be set to True manually when modifying that data. The session cookie will only be written to the response if this is True.

setdefault(k[, d]) → D.get(k,d), also set D[k]=d if k not in D
class flask.sessions.NullSession(initial=None)

Class used to generate nicer error messages if sessions are not available. Will still allow read-only access to the empty session but fail on setting.

class flask.sessions.SessionMixin

Expands a basic dictionary with session attributes.

accessed = True

Some implementations can detect when session data is read or written and set this when that happens. The mixin default is hard coded to True.

modified = True

Some implementations can detect changes to the session and set this when that happens. The mixin default is hard coded to True.

permanent

This reflects the '_permanent' key in the dict.

Notice

The PERMANENT_SESSION_LIFETIME config key can also be an integer starting with Flask 0.8. Either catch this down yourself or use the permanent_session_lifetime attribute on the app which converts the result to an integer automatically.

Test Client

class flask.testing.FlaskClient(*args, **kwargs)

Works like a regular Werkzeug test client but has some knowledge about how Flask works to defer the cleanup of the request context stack to the end of a with body when used in a with statement. For general information about how to use this class refer to werkzeug.test.Client.

Changelog

Changed in version 0.12: app.test_client() includes preset default environment, which can be set after instantiation of the app.test_client() object in client.environ_base.

Basic usage is outlined in the Testing Flask Applications chapter.

open(*args, **kwargs)

Takes the same arguments as the EnvironBuilder class with some additions: You can provide a EnvironBuilder or a WSGI environment as only argument instead of the EnvironBuilder arguments and two optional keyword arguments (as_tuple, buffered) that change the type of the return value or the way the application is executed.

Changelog

Changed in version 0.5: If a dict is provided as file in the dict for the data parameter the content type has to be called content_type now instead of mimetype. This change was made for consistency with werkzeug.FileWrapper.

The follow_redirects parameter was added to open().

Additional parameters:

Parameters:
  • as_tuple – Returns a tuple in the form (environ, result)
  • buffered – Set this to True to buffer the application run. This will automatically close the application for you as well.
  • follow_redirects – Set this to True if the Client should follow HTTP redirects.
session_transaction(*args, **kwargs)

When used in combination with a with statement this opens a session transaction. This can be used to modify the session that the test client uses. Once the with block is left the session is stored back.

with client.session_transaction() as session:
    session['value'] = 42

Internally this is implemented by going through a temporary test request context and since session handling could depend on request variables this function accepts the same arguments as test_request_context() which are directly passed through.

Test CLI Runner

class flask.testing.FlaskCliRunner(app, **kwargs)

A CliRunner for testing a Flask app’s CLI commands. Typically created using test_cli_runner(). See Testing CLI Commands.

invoke(cli=None, args=None, **kwargs)

Invokes a CLI command in an isolated environment. See CliRunner.invoke for full method documentation. See Testing CLI Commands for examples.

If the obj argument is not given, passes an instance of ScriptInfo that knows how to load the Flask app being tested.

Parameters:
  • cli – Command object to invoke. Default is the app’s cli group.
  • args – List of strings to invoke the command with.
Returns:

a Result object.

Application Globals

To share data that is valid for one request only from one function to another, a global variable is not good enough because it would break in threaded environments. Flask provides you with a special object that ensures it is only valid for the active request and that will return different values for each request. In a nutshell: it does the right thing, like it does for request and session.

flask.g

A namespace object that can store data during an application context. This is an instance of Flask.app_ctx_globals_class, which defaults to ctx._AppCtxGlobals.

This is a good place to store resources during a request. During testing, you can use the Faking Resources and Context pattern to pre-configure such resources.

This is a proxy. See Notes On Proxies for more information.

Changelog

Changed in version 0.10: Bound to the application context instead of the request context.

class flask.ctx._AppCtxGlobals

A plain object. Used as a namespace for storing data during an application context.

Creating an app context automatically creates this object, which is made available as the g proxy.

'key' in g

Check whether an attribute is present.

Changelog

New in version 0.10.

iter(g)

Return an iterator over the attribute names.

Changelog

New in version 0.10.

get(name, default=None)

Get an attribute by name, or a default value. Like dict.get().

Parameters:
  • name – Name of attribute to get.
  • default – Value to return if the attribute is not present.
Changelog

New in version 0.10.

pop(name, default=<object object>)

Get and remove an attribute by name. Like dict.pop().

Parameters:
  • name – Name of attribute to pop.
  • default – Value to return if the attribute is not present, instead of raise a KeyError.
Changelog

New in version 0.11.

setdefault(name, default=None)

Get the value of an attribute if it is present, otherwise set and return a default value. Like dict.setdefault().

Parameters:name – Name of attribute to get.
Param:default: Value to set and return if the attribute is not present.
Changelog

New in version 0.11.

Useful Functions and Classes

flask.current_app

A proxy to the application handling the current request. This is useful to access the application without needing to import it, or if it can’t be imported, such as when using the application factory pattern or in blueprints and extensions.

This is only available when an application context is pushed. This happens automatically during requests and CLI commands. It can be controlled manually with app_context().

This is a proxy. See Notes On Proxies for more information.

flask.has_request_context()

If you have code that wants to test if a request context is there or not this function can be used. For instance, you may want to take advantage of request information if the request object is available, but fail silently if it is unavailable.

class User(db.Model):

    def __init__(self, username, remote_addr=None):
        self.username = username
        if remote_addr is None and has_request_context():
            remote_addr = request.remote_addr
        self.remote_addr = remote_addr

Alternatively you can also just test any of the context bound objects (such as request or g for truthness):

class User(db.Model):

    def __init__(self, username, remote_addr=None):
        self.username = username
        if remote_addr is None and request:
            remote_addr = request.remote_addr
        self.remote_addr = remote_addr
Changelog

New in version 0.7.

flask.copy_current_request_context(f)

A helper function that decorates a function to retain the current request context. This is useful when working with greenlets. The moment the function is decorated a copy of the request context is created and then pushed when the function is called.

Example:

import gevent
from flask import copy_current_request_context

@app.route('/')
def index():
    @copy_current_request_context
    def do_some_work():
        # do some work here, it can access flask.request like you
        # would otherwise in the view function.
        ...
    gevent.spawn(do_some_work)
    return 'Regular response'
Changelog

New in version 0.10.

flask.has_app_context()

Works like has_request_context() but for the application context. You can also just do a boolean check on the current_app object instead.

Changelog

New in version 0.9.

flask.url_for(endpoint, **values)

Generates a URL to the given endpoint with the method provided.

Variable arguments that are unknown to the target endpoint are appended to the generated URL as query arguments. If the value of a query argument is None, the whole pair is skipped. In case blueprints are active you can shortcut references to the same blueprint by prefixing the local endpoint with a dot (.).

This will reference the index function local to the current blueprint:

url_for('.index')

For more information, head over to the Quickstart.

To integrate applications, Flask has a hook to intercept URL build errors through Flask.url_build_error_handlers. The url_for function results in a BuildError when the current app does not have a URL for the given endpoint and values. When it does, the current_app calls its url_build_error_handlers if it is not None, which can return a string to use as the result of url_for (instead of url_for’s default to raise the BuildError exception) or re-raise the exception. An example:

def external_url_handler(error, endpoint, values):
    "Looks up an external URL when `url_for` cannot build a URL."
    # This is an example of hooking the build_error_handler.
    # Here, lookup_url is some utility function you've built
    # which looks up the endpoint in some external URL registry.
    url = lookup_url(endpoint, **values)
    if url is None:
        # External lookup did not have a URL.
        # Re-raise the BuildError, in context of original traceback.
        exc_type, exc_value, tb = sys.exc_info()
        if exc_value is error:
            raise exc_type, exc_value, tb
        else:
            raise error
    # url_for will use this result, instead of raising BuildError.
    return url

app.url_build_error_handlers.append(external_url_handler)

Here, error is the instance of BuildError, and endpoint and values are the arguments passed into url_for. Note that this is for building URLs outside the current application, and not for handling 404 NotFound errors.

Changelog

New in version 0.10: The _scheme parameter was added.

New in version 0.9: The _anchor and _method parameters were added.

New in version 0.9: Calls Flask.handle_build_error() on BuildError.

Parameters:
  • endpoint – the endpoint of the URL (name of the function)
  • values – the variable arguments of the URL rule
  • _external – if set to True, an absolute URL is generated. Server address can be changed via SERVER_NAME configuration variable which defaults to localhost.
  • _scheme – a string specifying the desired URL scheme. The _external parameter must be set to True or a ValueError is raised. The default behavior uses the same scheme as the current request, or PREFERRED_URL_SCHEME from the app configuration if no request context is available. As of Werkzeug 0.10, this also can be set to an empty string to build protocol-relative URLs.
  • _anchor – if provided this is added as anchor to the URL.
  • _method – if provided this explicitly specifies an HTTP method.
flask.abort(status, *args, **kwargs)

Raises an HTTPException for the given status code or WSGI application:

abort(404)  # 404 Not Found
abort(Response('Hello World'))

Can be passed a WSGI application or a status code. If a status code is given it’s looked up in the list of exceptions and will raise that exception, if passed a WSGI application it will wrap it in a proxy WSGI exception and raise that:

abort(404)
abort(Response('Hello World'))
flask.redirect(location, code=302, Response=None)

Returns a response object (a WSGI application) that, if called, redirects the client to the target location. Supported codes are 301, 302, 303, 305, and 307. 300 is not supported because it’s not a real redirect and 304 because it’s the answer for a request with a request with defined If-Modified-Since headers.

Changelog

New in version 0.10: The class used for the Response object can now be passed in.

New in version 0.6: The location can now be a unicode string that is encoded using the iri_to_uri() function.

Parameters:
  • location – the location the response should redirect to.
  • code – the redirect status code. defaults to 302.
  • Response (class) – a Response class to use when instantiating a response. The default is werkzeug.wrappers.Response if unspecified.
flask.make_response(*args)

Sometimes it is necessary to set additional headers in a view. Because views do not have to return response objects but can return a value that is converted into a response object by Flask itself, it becomes tricky to add headers to it. This function can be called instead of using a return and you will get a response object which you can use to attach headers.

If view looked like this and you want to add a new header:

def index():
    return render_template('index.html', foo=42)

You can now do something like this:

def index():
    response = make_response(render_template('index.html', foo=42))
    response.headers['X-Parachutes'] = 'parachutes are cool'
    return response

This function accepts the very same arguments you can return from a view function. This for example creates a response with a 404 error code:

response = make_response(render_template('not_found.html'), 404)

The other use case of this function is to force the return value of a view function into a response which is helpful with view decorators:

response = make_response(view_function())
response.headers['X-Parachutes'] = 'parachutes are cool'

Internally this function does the following things:

Changelog

New in version 0.6.

flask.after_this_request(f)

Executes a function after this request. This is useful to modify response objects. The function is passed the response object and has to return the same or a new one.

Example:

@app.route('/')
def index():
    @after_this_request
    def add_header(response):
        response.headers['X-Foo'] = 'Parachute'
        return response
    return 'Hello World!'

This is more useful if a function other than the view function wants to modify a response. For instance think of a decorator that wants to add some headers without converting the return value into a response object.

Changelog

New in version 0.9.

flask.send_file(filename_or_fp, mimetype=None, as_attachment=False, attachment_filename=None, add_etags=True, cache_timeout=None, conditional=False, last_modified=None)

Sends the contents of a file to the client. This will use the most efficient method available and configured. By default it will try to use the WSGI server’s file_wrapper support. Alternatively you can set the application’s use_x_sendfile attribute to True to directly emit an X-Sendfile header. This however requires support of the underlying webserver for X-Sendfile.

By default it will try to guess the mimetype for you, but you can also explicitly provide one. For extra security you probably want to send certain files as attachment (HTML for instance). The mimetype guessing requires a filename or an attachment_filename to be provided.

ETags will also be attached automatically if a filename is provided. You can turn this off by setting add_etags=False.

If conditional=True and filename is provided, this method will try to upgrade the response stream to support range requests. This will allow the request to be answered with partial content response.

Please never pass filenames to this function from user sources; you should use send_from_directory() instead.

Changelog

Changed in version 1.0: UTF-8 filenames, as specified in RFC 2231, are supported.

Changed in version 0.12: The filename is no longer automatically inferred from file objects. If you want to use automatic mimetype and etag support, pass a filepath via filename_or_fp or attachment_filename.

Changed in version 0.12: The attachment_filename is preferred over filename for MIME-type detection.

Changed in version 0.9: cache_timeout pulls its default from application config, when None.

Changed in version 0.7: mimetype guessing and etag support for file objects was deprecated because it was unreliable. Pass a filename if you are able to, otherwise attach an etag yourself. This functionality will be removed in Flask 1.0

New in version 0.5: The add_etags, cache_timeout and conditional parameters were added. The default behavior is now to attach etags.

New in version 0.2.

Parameters:
  • filename_or_fp – the filename of the file to send. This is relative to the root_path if a relative path is specified. Alternatively a file object might be provided in which case X-Sendfile might not work and fall back to the traditional method. Make sure that the file pointer is positioned at the start of data to send before calling send_file().
  • mimetype – the mimetype of the file if provided. If a file path is given, auto detection happens as fallback, otherwise an error will be raised.
  • as_attachment – set to True if you want to send this file with a Content-Disposition: attachment header.
  • attachment_filename – the filename for the attachment if it differs from the file’s filename.
  • add_etags – set to False to disable attaching of etags.
  • conditional – set to True to enable conditional responses.
  • cache_timeout – the timeout in seconds for the headers. When None (default), this value is set by get_send_file_max_age() of current_app.
  • last_modified – set the Last-Modified header to this value, a datetime or timestamp. If a file was passed, this overrides its mtime.
flask.send_from_directory(directory, filename, **options)

Send a file from a given directory with send_file(). This is a secure way to quickly expose static files from an upload folder or something similar.

Example usage:

@app.route('/uploads/<path:filename>')
def download_file(filename):
    return send_from_directory(app.config['UPLOAD_FOLDER'],
                               filename, as_attachment=True)

Sending files and Performance

It is strongly recommended to activate either X-Sendfile support in your webserver or (if no authentication happens) to tell the webserver to serve files for the given path on its own without calling into the web application for improved performance.

Changelog

New in version 0.5.

Parameters:
  • directory – the directory where all the files are stored.
  • filename – the filename relative to that directory to download.
  • options – optional keyword arguments that are directly forwarded to send_file().
flask.safe_join(directory, *pathnames)

Safely join directory and zero or more untrusted pathnames components.

Example usage:

@app.route('/wiki/<path:filename>')
def wiki_page(filename):
    filename = safe_join(app.config['WIKI_FOLDER'], filename)
    with open(filename, 'rb') as fd:
        content = fd.read()  # Read and process the file content...
Parameters:
  • directory – the trusted base directory.
  • pathnames – the untrusted pathnames relative to that directory.
Raises:

NotFound if one or more passed paths fall out of its boundaries.

flask.escape(s) → markup

Convert the characters &, <, >, ‘, and ” in string s to HTML-safe sequences. Use this if you need to display text that might contain such characters in HTML. Marks return value as markup string.

class flask.Markup

Marks a string as being safe for inclusion in HTML/XML output without needing to be escaped. This implements the __html__ interface a couple of frameworks and web applications use. Markup is a direct subclass of unicode and provides all the methods of unicode just that it escapes arguments passed and always returns Markup.

The escape function returns markup objects so that double escaping can’t happen.

The constructor of the Markup class can be used for three different things: When passed an unicode object it’s assumed to be safe, when passed an object with an HTML representation (has an __html__ method) that representation is used, otherwise the object passed is converted into a unicode string and then assumed to be safe:

>>> Markup("Hello <em>World</em>!")
Markup(u'Hello <em>World</em>!')
>>> class Foo(object):
...  def __html__(self):
...   return '<a href="#">foo</a>'
...
>>> Markup(Foo())
Markup(u'<a href="#">foo</a>')

If you want object passed being always treated as unsafe you can use the escape() classmethod to create a Markup object:

>>> Markup.escape("Hello <em>World</em>!")
Markup(u'Hello &lt;em&gt;World&lt;/em&gt;!')

Operations on a markup string are markup aware which means that all arguments are passed through the escape() function:

>>> em = Markup("<em>%s</em>")
>>> em % "foo & bar"
Markup(u'<em>foo &amp; bar</em>')
>>> strong = Markup("<strong>%(text)s</strong>")
>>> strong % {'text': '<blink>hacker here</blink>'}
Markup(u'<strong>&lt;blink&gt;hacker here&lt;/blink&gt;</strong>')
>>> Markup("<em>Hello</em> ") + "<foo>"
Markup(u'<em>Hello</em> &lt;foo&gt;')
classmethod escape(s)

Escape the string. Works like escape() with the difference that for subclasses of Markup this function would return the correct subclass.

striptags()

Unescape markup into an text_type string and strip all tags. This also resolves known HTML4 and XHTML entities. Whitespace is normalized to one:

>>> Markup("Main &raquo;  <em>About</em>").striptags()
u'Main \xbb About'
unescape()

Unescape markup again into an text_type string. This also resolves known HTML4 and XHTML entities:

>>> Markup("Main &raquo; <em>About</em>").unescape()
u'Main \xbb <em>About</em>'

Message Flashing

flask.flash(message, category='message')

Flashes a message to the next request. In order to remove the flashed message from the session and to display it to the user, the template has to call get_flashed_messages().

Changelog

Changed in version 0.3: category parameter added.

Parameters:
  • message – the message to be flashed.
  • category – the category for the message. The following values are recommended: 'message' for any kind of message, 'error' for errors, 'info' for information messages and 'warning' for warnings. However any kind of string can be used as category.
flask.get_flashed_messages(with_categories=False, category_filter=[])

Pulls all flashed messages from the session and returns them. Further calls in the same request to the function will return the same messages. By default just the messages are returned, but when with_categories is set to True, the return value will be a list of tuples in the form (category, message) instead.

Filter the flashed messages to one or more categories by providing those categories in category_filter. This allows rendering categories in separate html blocks. The with_categories and category_filter arguments are distinct:

  • with_categories controls whether categories are returned with message text (True gives a tuple, where False gives just the message text).
  • category_filter filters the messages down to only those matching the provided categories.

See Message Flashing for examples.

Changelog

Changed in version 0.9: category_filter parameter added.

Changed in version 0.3: with_categories parameter added.

Parameters:
  • with_categories – set to True to also receive categories.
  • category_filter – whitelist of categories to limit return values

JSON Support

Flask uses simplejson for the JSON implementation. Since simplejson is provided by both the standard library as well as extension, Flask will try simplejson first and then fall back to the stdlib json module. On top of that it will delegate access to the current application’s JSON encoders and decoders for easier customization.

So for starters instead of doing:

try:
    import simplejson as json
except ImportError:
    import json

You can instead just do this:

from flask import json

For usage examples, read the json documentation in the standard library. The following extensions are by default applied to the stdlib’s JSON module:

  1. datetime objects are serialized as RFC 822 strings.
  2. Any object with an __html__ method (like Markup) will have that method called and then the return value is serialized as string.

The htmlsafe_dumps() function of this json module is also available as filter called |tojson in Jinja2. Note that inside script tags no escaping must take place, so make sure to disable escaping with |safe if you intend to use it inside script tags unless you are using Flask 0.10 which implies that:

<script type=text/javascript>
    doSomethingWith({{ user.username|tojson|safe }});
</script>

Auto-Sort JSON Keys

The configuration variable JSON_SORT_KEYS (Configuration Handling) can be set to false to stop Flask from auto-sorting keys. By default sorting is enabled and outside of the app context sorting is turned on.

Notice that disabling key sorting can cause issues when using content based HTTP caches and Python’s hash randomization feature.

flask.json.jsonify(*args, **kwargs)

This function wraps dumps() to add a few enhancements that make life easier. It turns the JSON output into a Response object with the application/json mimetype. For convenience, it also converts multiple arguments into an array or multiple keyword arguments into a dict. This means that both jsonify(1,2,3) and jsonify([1,2,3]) serialize to [1,2,3].

For clarity, the JSON serialization behavior has the following differences from dumps():

  1. Single argument: Passed straight through to dumps().
  2. Multiple arguments: Converted to an array before being passed to dumps().
  3. Multiple keyword arguments: Converted to a dict before being passed to dumps().
  4. Both args and kwargs: Behavior undefined and will throw an exception.

Example usage:

from flask import jsonify

@app.route('/_get_current_user')
def get_current_user():
    return jsonify(username=g.user.username,
                   email=g.user.email,
                   id=g.user.id)

This will send a JSON response like this to the browser:

{
    "username": "admin",
    "email": "admin@localhost",
    "id": 42
}
Changelog

Changed in version 0.11: Added support for serializing top-level arrays. This introduces a security risk in ancient browsers. See JSON Security for details.

This function’s response will be pretty printed if the JSONIFY_PRETTYPRINT_REGULAR config parameter is set to True or the Flask app is running in debug mode. Compressed (not pretty) formatting currently means no indents and no spaces after separators.

Changelog

New in version 0.2.

flask.json.dumps(obj, **kwargs)

Serialize obj to a JSON formatted str by using the application’s configured encoder (json_encoder) if there is an application on the stack.

This function can return unicode strings or ascii-only bytestrings by default which coerce into unicode strings automatically. That behavior by default is controlled by the JSON_AS_ASCII configuration variable and can be overridden by the simplejson ensure_ascii parameter.

flask.json.dump(obj, fp, **kwargs)

Like dumps() but writes into a file object.

flask.json.loads(s, **kwargs)

Unserialize a JSON object from a string s by using the application’s configured decoder (json_decoder) if there is an application on the stack.

flask.json.load(fp, **kwargs)

Like loads() but reads from a file object.

class flask.json.JSONEncoder(*, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, sort_keys=False, indent=None, separators=None, default=None)

The default Flask JSON encoder. This one extends the default simplejson encoder by also supporting datetime objects, UUID as well as Markup objects which are serialized as RFC 822 datetime strings (same as the HTTP date format). In order to support more data types override the default() method.

default(o)

Implement this method in a subclass such that it returns a serializable object for o, or calls the base implementation (to raise a TypeError).

For example, to support arbitrary iterators, you could implement default like this:

def default(self, o):
    try:
        iterable = iter(o)
    except TypeError:
        pass
    else:
        return list(iterable)
    return JSONEncoder.default(self, o)
class flask.json.JSONDecoder(*, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, strict=True, object_pairs_hook=None)

The default JSON decoder. This one does not change the behavior from the default simplejson decoder. Consult the json documentation for more information. This decoder is not only used for the load functions of this module but also Request.

Tagged JSON

A compact representation for lossless serialization of non-standard JSON types. SecureCookieSessionInterface uses this to serialize the session data, but it may be useful in other places. It can be extended to support other types.

class flask.json.tag.TaggedJSONSerializer

Serializer that uses a tag system to compactly represent objects that are not JSON types. Passed as the intermediate serializer to itsdangerous.Serializer.

The following extra types are supported:

default_tags = [<class 'flask.json.tag.TagDict'>, <class 'flask.json.tag.PassDict'>, <class 'flask.json.tag.TagTuple'>, <class 'flask.json.tag.PassList'>, <class 'flask.json.tag.TagBytes'>, <class 'flask.json.tag.TagMarkup'>, <class 'flask.json.tag.TagUUID'>, <class 'flask.json.tag.TagDateTime'>]

Tag classes to bind when creating the serializer. Other tags can be added later using register().

dumps(value)

Tag the value and dump it to a compact JSON string.

loads(value)

Load data from a JSON string and deserialized any tagged objects.

register(tag_class, force=False, index=None)

Register a new tag with this serializer.

Parameters:
  • tag_class – tag class to register. Will be instantiated with this serializer instance.
  • force – overwrite an existing tag. If false (default), a KeyError is raised.
  • index – index to insert the new tag in the tag order. Useful when the new tag is a special case of an existing tag. If None (default), the tag is appended to the end of the order.
Raises:

KeyError – if the tag key is already registered and force is not true.

tag(value)

Convert a value to a tagged representation if necessary.

untag(value)

Convert a tagged representation back to the original type.

class flask.json.tag.JSONTag(serializer)

Base class for defining type tags for TaggedJSONSerializer.

check(value)

Check if the given value should be tagged by this tag.

key = None

The tag to mark the serialized object with. If None, this tag is only used as an intermediate step during tagging.

tag(value)

Convert the value to a valid JSON type and add the tag structure around it.

to_json(value)

Convert the Python object to an object that is a valid JSON type. The tag will be added later.

to_python(value)

Convert the JSON representation back to the correct type. The tag will already be removed.

Let’s seen an example that adds support for OrderedDict. Dicts don’t have an order in Python or JSON, so to handle this we will dump the items as a list of [key, value] pairs. Subclass JSONTag and give it the new key ' od' to identify the type. The session serializer processes dicts first, so insert the new tag at the front of the order since OrderedDict must be processed before dict.

from flask.json.tag import JSONTag

class TagOrderedDict(JSONTag):
    __slots__ = ('serializer',)
    key = ' od'

    def check(self, value):
        return isinstance(value, OrderedDict)

    def to_json(self, value):
        return [[k, self.serializer.tag(v)] for k, v in iteritems(value)]

    def to_python(self, value):
        return OrderedDict(value)

app.session_interface.serializer.register(TagOrderedDict, index=0)

Template Rendering

flask.render_template(template_name_or_list, **context)

Renders a template from the template folder with the given context.

Parameters:
  • template_name_or_list – the name of the template to be rendered, or an iterable with template names the first one existing will be rendered
  • context – the variables that should be available in the context of the template.
flask.render_template_string(source, **context)

Renders a template from the given template source string with the given context. Template variables will be autoescaped.

Parameters:
  • source – the source code of the template to be rendered
  • context – the variables that should be available in the context of the template.
flask.get_template_attribute(template_name, attribute)

Loads a macro (or variable) a template exports. This can be used to invoke a macro from within Python code. If you for example have a template named _cider.html with the following contents:

{% macro hello(name) %}Hello {{ name }}!{% endmacro %}

You can access this from Python code like this:

hello = get_template_attribute('_cider.html', 'hello')
return hello('World')
Changelog

New in version 0.2.

Parameters:
  • template_name – the name of the template
  • attribute – the name of the variable of macro to access

Configuration

class flask.Config(root_path, defaults=None)

Works exactly like a dict but provides ways to fill it from files or special dictionaries. There are two common patterns to populate the config.

Either you can fill the config from a config file:

app.config.from_pyfile('yourconfig.cfg')

Or alternatively you can define the configuration options in the module that calls from_object() or provide an import path to a module that should be loaded. It is also possible to tell it to use the same module and with that provide the configuration values just before the call:

DEBUG = True
SECRET_KEY = 'development key'
app.config.from_object(__name__)

In both cases (loading from any Python file or loading from modules), only uppercase keys are added to the config. This makes it possible to use lowercase values in the config file for temporary values that are not added to the config or to define the config keys in the same file that implements the application.

Probably the most interesting way to load configurations is from an environment variable pointing to a file:

app.config.from_envvar('YOURAPPLICATION_SETTINGS')

In this case before launching the application you have to set this environment variable to the file you want to use. On Linux and OS X use the export statement:

export YOURAPPLICATION_SETTINGS='/path/to/config/file'

On windows use set instead.

Parameters:
  • root_path – path to which files are read relative from. When the config object is created by the application, this is the application’s root_path.
  • defaults – an optional dictionary of default values
from_envvar(variable_name, silent=False)

Loads a configuration from an environment variable pointing to a configuration file. This is basically just a shortcut with nicer error messages for this line of code:

app.config.from_pyfile(os.environ['YOURAPPLICATION_SETTINGS'])
Parameters:
  • variable_name – name of the environment variable
  • silent – set to True if you want silent failure for missing files.
Returns:

bool. True if able to load config, False otherwise.

from_json(filename, silent=False)

Updates the values in the config from a JSON file. This function behaves as if the JSON object was a dictionary and passed to the from_mapping() function.

Parameters:
  • filename – the filename of the JSON file. This can either be an absolute filename or a filename relative to the root path.
  • silent – set to True if you want silent failure for missing files.
Changelog

New in version 0.11.

from_mapping(*mapping, **kwargs)

Updates the config like update() ignoring items with non-upper keys.

Changelog

New in version 0.11.

from_object(obj)

Updates the values from the given object. An object can be of one of the following two types:

  • a string: in this case the object with that name will be imported
  • an actual object reference: that object is used directly

Objects are usually either modules or classes. from_object() loads only the uppercase attributes of the module/class. A dict object will not work with from_object() because the keys of a dict are not attributes of the dict class.

Example of module-based configuration:

app.config.from_object('yourapplication.default_config')
from yourapplication import default_config
app.config.from_object(default_config)

You should not use this function to load the actual configuration but rather configuration defaults. The actual config should be loaded with from_pyfile() and ideally from a location not within the package because the package might be installed system wide.

See Development / Production for an example of class-based configuration using from_object().

Parameters:obj – an import name or object
from_pyfile(filename, silent=False)

Updates the values in the config from a Python file. This function behaves as if the file was imported as module with the from_object() function.

Parameters:
  • filename – the filename of the config. This can either be an absolute filename or a filename relative to the root path.
  • silent – set to True if you want silent failure for missing files.
Changelog

New in version 0.7: silent parameter.

get_namespace(namespace, lowercase=True, trim_namespace=True)

Returns a dictionary containing a subset of configuration options that match the specified namespace/prefix. Example usage:

app.config['IMAGE_STORE_TYPE'] = 'fs'
app.config['IMAGE_STORE_PATH'] = '/var/app/images'
app.config['IMAGE_STORE_BASE_URL'] = 'http://img.website.com'
image_store_config = app.config.get_namespace('IMAGE_STORE_')

The resulting dictionary image_store_config would look like:

{
    'type': 'fs',
    'path': '/var/app/images',
    'base_url': 'http://img.website.com'
}

This is often useful when configuration options map directly to keyword arguments in functions or class constructors.

Parameters:
  • namespace – a configuration namespace
  • lowercase – a flag indicating if the keys of the resulting dictionary should be lowercase
  • trim_namespace – a flag indicating if the keys of the resulting dictionary should not include the namespace
Changelog

New in version 0.11.

Stream Helpers

flask.stream_with_context(generator_or_function)

Request contexts disappear when the response is started on the server. This is done for efficiency reasons and to make it less likely to encounter memory leaks with badly written WSGI middlewares. The downside is that if you are using streamed responses, the generator cannot access request bound information any more.

This function however can help you keep the context around for longer:

from flask import stream_with_context, request, Response

@app.route('/stream')
def streamed_response():
    @stream_with_context
    def generate():
        yield 'Hello '
        yield request.args['name']
        yield '!'
    return Response(generate())

Alternatively it can also be used around a specific generator:

from flask import stream_with_context, request, Response

@app.route('/stream')
def streamed_response():
    def generate():
        yield 'Hello '
        yield request.args['name']
        yield '!'
    return Response(stream_with_context(generate()))
Changelog

New in version 0.9.

Useful Internals

class flask.ctx.RequestContext(app, environ, request=None)

The request context contains all request relevant information. It is created at the beginning of the request and pushed to the _request_ctx_stack and removed at the end of it. It will create the URL adapter and request object for the WSGI environment provided.

Do not attempt to use this class directly, instead use test_request_context() and request_context() to create this object.

When the request context is popped, it will evaluate all the functions registered on the application for teardown execution (teardown_request()).

The request context is automatically popped at the end of the request for you. In debug mode the request context is kept around if exceptions happen so that interactive debuggers have a chance to introspect the data. With 0.4 this can also be forced for requests that did not fail and outside of DEBUG mode. By setting 'flask._preserve_context' to True on the WSGI environment the context will not pop itself at the end of the request. This is used by the test_client() for example to implement the deferred cleanup functionality.

You might find this helpful for unittests where you need the information from the context local around for a little longer. Make sure to properly pop() the stack yourself in that situation, otherwise your unittests will leak memory.

copy()

Creates a copy of this request context with the same request object. This can be used to move a request context to a different greenlet. Because the actual request object is the same this cannot be used to move a request context to a different thread unless access to the request object is locked.

Changelog

New in version 0.10.

match_request()

Can be overridden by a subclass to hook into the matching of the request.

pop(exc=<object object>)

Pops the request context and unbinds it by doing that. This will also trigger the execution of functions registered by the teardown_request() decorator.

Changelog

Changed in version 0.9: Added the exc argument.

push()

Binds the request context to the current context.

flask._request_ctx_stack

The internal LocalStack that holds RequestContext instances. Typically, the request and session proxies should be accessed instead of the stack. It may be useful to access the stack in extension code.

The following attributes are always present on each layer of the stack:

app
the active Flask application.
url_adapter
the URL adapter that was used to match the request.
request
the current request object.
session
the active session object.
g
an object with all the attributes of the flask.g object.
flashes
an internal cache for the flashed messages.

Example usage:

from flask import _request_ctx_stack

def get_session():
    ctx = _request_ctx_stack.top
    if ctx is not None:
        return ctx.session
class flask.ctx.AppContext(app)

The application context binds an application object implicitly to the current thread or greenlet, similar to how the RequestContext binds request information. The application context is also implicitly created if a request context is created but the application is not on top of the individual application context.

pop(exc=<object object>)

Pops the app context.

push()

Binds the app context to the current context.

flask._app_ctx_stack

The internal LocalStack that holds AppContext instances. Typically, the current_app and g proxies should be accessed instead of the stack. Extensions can access the contexts on the stack as a namespace to store data.

Changelog

New in version 0.9.

class flask.blueprints.BlueprintSetupState(blueprint, app, options, first_registration)

Temporary holder object for registering a blueprint with the application. An instance of this class is created by the make_setup_state() method and later passed to all register callback functions.

add_url_rule(rule, endpoint=None, view_func=None, **options)

A helper method to register a rule (and optionally a view function) to the application. The endpoint is automatically prefixed with the blueprint’s name.

app = None

a reference to the current application

blueprint = None

a reference to the blueprint that created this setup state.

first_registration = None

as blueprints can be registered multiple times with the application and not everything wants to be registered multiple times on it, this attribute can be used to figure out if the blueprint was registered in the past already.

options = None

a dictionary with all options that were passed to the register_blueprint() method.

subdomain = None

The subdomain that the blueprint should be active for, None otherwise.

url_defaults = None

A dictionary with URL defaults that is added to each and every URL that was defined with the blueprint.

url_prefix = None

The prefix that should be used for all URLs defined on the blueprint.

Signals

Changelog

New in version 0.6.

signals.signals_available

True if the signaling system is available. This is the case when blinker is installed.

The following signals exist in Flask:

flask.template_rendered

This signal is sent when a template was successfully rendered. The signal is invoked with the instance of the template as template and the context as dictionary (named context).

Example subscriber:

def log_template_renders(sender, template, context, **extra):
    sender.logger.debug('Rendering template "%s" with context %s',
                        template.name or 'string template',
                        context)

from flask import template_rendered
template_rendered.connect(log_template_renders, app)
flask.before_render_template

This signal is sent before template rendering process. The signal is invoked with the instance of the template as template and the context as dictionary (named context).

Example subscriber:

def log_template_renders(sender, template, context, **extra):
    sender.logger.debug('Rendering template "%s" with context %s',
                        template.name or 'string template',
                        context)

from flask import before_render_template
before_render_template.connect(log_template_renders, app)
flask.request_started

This signal is sent when the request context is set up, before any request processing happens. Because the request context is already bound, the subscriber can access the request with the standard global proxies such as request.

Example subscriber:

def log_request(sender, **extra):
    sender.logger.debug('Request context is set up')

from flask import request_started
request_started.connect(log_request, app)
flask.request_finished

This signal is sent right before the response is sent to the client. It is passed the response to be sent named response.

Example subscriber:

def log_response(sender, response, **extra):
    sender.logger.debug('Request context is about to close down.  '
                        'Response: %s', response)

from flask import request_finished
request_finished.connect(log_response, app)
flask.got_request_exception

This signal is sent when an exception happens during request processing. It is sent before the standard exception handling kicks in and even in debug mode, where no exception handling happens. The exception itself is passed to the subscriber as exception.

Example subscriber:

def log_exception(sender, exception, **extra):
    sender.logger.debug('Got exception during processing: %s', exception)

from flask import got_request_exception
got_request_exception.connect(log_exception, app)
flask.request_tearing_down

This signal is sent when the request is tearing down. This is always called, even if an exception is caused. Currently functions listening to this signal are called after the regular teardown handlers, but this is not something you can rely on.

Example subscriber:

def close_db_connection(sender, **extra):
    session.close()

from flask import request_tearing_down
request_tearing_down.connect(close_db_connection, app)

As of Flask 0.9, this will also be passed an exc keyword argument that has a reference to the exception that caused the teardown if there was one.

flask.appcontext_tearing_down

This signal is sent when the app context is tearing down. This is always called, even if an exception is caused. Currently functions listening to this signal are called after the regular teardown handlers, but this is not something you can rely on.

Example subscriber:

def close_db_connection(sender, **extra):
    session.close()

from flask import appcontext_tearing_down
appcontext_tearing_down.connect(close_db_connection, app)

This will also be passed an exc keyword argument that has a reference to the exception that caused the teardown if there was one.

flask.appcontext_pushed

This signal is sent when an application context is pushed. The sender is the application. This is usually useful for unittests in order to temporarily hook in information. For instance it can be used to set a resource early onto the g object.

Example usage:

from contextlib import contextmanager
from flask import appcontext_pushed

@contextmanager
def user_set(app, user):
    def handler(sender, **kwargs):
        g.user = user
    with appcontext_pushed.connected_to(handler, app):
        yield

And in the testcode:

def test_user_me(self):
    with user_set(app, 'john'):
        c = app.test_client()
        resp = c.get('/users/me')
        assert resp.data == 'username=john'
Changelog

New in version 0.10.

flask.appcontext_popped

This signal is sent when an application context is popped. The sender is the application. This usually falls in line with the appcontext_tearing_down signal.

Changelog

New in version 0.10.

flask.message_flashed

This signal is sent when the application is flashing a message. The messages is sent as message keyword argument and the category as category.

Example subscriber:

recorded = []
def record(sender, message, category, **extra):
    recorded.append((message, category))

from flask import message_flashed
message_flashed.connect(record, app)
Changelog

New in version 0.10.

class signals.Namespace

An alias for blinker.base.Namespace if blinker is available, otherwise a dummy class that creates fake signals. This class is available for Flask extensions that want to provide the same fallback system as Flask itself.

signal(name, doc=None)

Creates a new signal for this namespace if blinker is available, otherwise returns a fake signal that has a send method that will do nothing but will fail with a RuntimeError for all other operations, including connecting.

Class-Based Views

Changelog

New in version 0.7.

class flask.views.View

Alternative way to use view functions. A subclass has to implement dispatch_request() which is called with the view arguments from the URL routing system. If methods is provided the methods do not have to be passed to the add_url_rule() method explicitly:

class MyView(View):
    methods = ['GET']

    def dispatch_request(self, name):
        return 'Hello %s!' % name

app.add_url_rule('/hello/<name>', view_func=MyView.as_view('myview'))

When you want to decorate a pluggable view you will have to either do that when the view function is created (by wrapping the return value of as_view()) or you can use the decorators attribute:

class SecretView(View):
    methods = ['GET']
    decorators = [superuser_required]

    def dispatch_request(self):
        ...

The decorators stored in the decorators list are applied one after another when the view function is created. Note that you can not use the class based decorators since those would decorate the view class and not the generated view function!

classmethod as_view(name, *class_args, **class_kwargs)

Converts the class into an actual view function that can be used with the routing system. Internally this generates a function on the fly which will instantiate the View on each request and call the dispatch_request() method on it.

The arguments passed to as_view() are forwarded to the constructor of the class.

decorators = ()

The canonical way to decorate class-based views is to decorate the return value of as_view(). However since this moves parts of the logic from the class declaration to the place where it’s hooked into the routing system.

You can place one or more decorators in this list and whenever the view function is created the result is automatically decorated.

Changelog

New in version 0.8.

dispatch_request()

Subclasses have to override this method to implement the actual view function code. This method is called with all the arguments from the URL rule.

methods = None

A list of methods this view can handle.

provide_automatic_options = None

Setting this disables or force-enables the automatic options handling.

class flask.views.MethodView

A class-based view that dispatches request methods to the corresponding class methods. For example, if you implement a get method, it will be used to handle GET requests.

class CounterAPI(MethodView):
    def get(self):
        return session.get('counter', 0)

    def post(self):
        session['counter'] = session.get('counter', 0) + 1
        return 'OK'

app.add_url_rule('/counter', view_func=CounterAPI.as_view('counter'))
dispatch_request(*args, **kwargs)

Subclasses have to override this method to implement the actual view function code. This method is called with all the arguments from the URL rule.

URL Route Registrations

Generally there are three ways to define rules for the routing system:

  1. You can use the flask.Flask.route() decorator.
  2. You can use the flask.Flask.add_url_rule() function.
  3. You can directly access the underlying Werkzeug routing system which is exposed as flask.Flask.url_map.

Variable parts in the route can be specified with angular brackets (/user/<username>). By default a variable part in the URL accepts any string without a slash however a different converter can be specified as well by using <converter:name>.

Variable parts are passed to the view function as keyword arguments.

The following converters are available:

string accepts any text without a slash (the default)
int accepts integers
float like int but for floating point values
path like the default but also accepts slashes
any matches one of the items provided
uuid accepts UUID strings

Custom converters can be defined using flask.Flask.url_map.

Here are some examples:

@app.route('/')
def index():
    pass

@app.route('/<username>')
def show_user(username):
    pass

@app.route('/post/<int:post_id>')
def show_post(post_id):
    pass

An important detail to keep in mind is how Flask deals with trailing slashes. The idea is to keep each URL unique so the following rules apply:

  1. If a rule ends with a slash and is requested without a slash by the user, the user is automatically redirected to the same page with a trailing slash attached.
  2. If a rule does not end with a trailing slash and the user requests the page with a trailing slash, a 404 not found is raised.

This is consistent with how web servers deal with static files. This also makes it possible to use relative link targets safely.

You can also define multiple rules for the same function. They have to be unique however. Defaults can also be specified. Here for example is a definition for a URL that accepts an optional page:

@app.route('/users/', defaults={'page': 1})
@app.route('/users/page/<int:page>')
def show_users(page):
    pass

This specifies that /users/ will be the URL for page one and /users/page/N will be the URL for page N.

If a URL contains a default value, it will be redirected to its simpler form with a 301 redirect. In the above example, /users/page/1 will be redirected to /users/. If your route handles GET and POST requests, make sure the default route only handles GET, as redirects can’t preserve form data.

@app.route('/region/', defaults={'id': 1})
@app.route('/region/<id>', methods=['GET', 'POST'])
def region(id):
   pass

Here are the parameters that route() and add_url_rule() accept. The only difference is that with the route parameter the view function is defined with the decorator instead of the view_func parameter.

rule the URL rule as string
endpoint the endpoint for the registered URL rule. Flask itself assumes that the name of the view function is the name of the endpoint if not explicitly stated.
view_func the function to call when serving a request to the provided endpoint. If this is not provided one can specify the function later by storing it in the view_functions dictionary with the endpoint as key.
defaults A dictionary with defaults for this rule. See the example above for how defaults work.
subdomain specifies the rule for the subdomain in case subdomain matching is in use. If not specified the default subdomain is assumed.
**options the options to be forwarded to the underlying Rule object. A change to Werkzeug is handling of method options. methods is a list of methods this rule should be limited to (GET, POST etc.). By default a rule just listens for GET (and implicitly HEAD). Starting with Flask 0.6, OPTIONS is implicitly added and handled by the standard request handling. They have to be specified as keyword arguments.

View Function Options

For internal usage the view functions can have some attributes attached to customize behavior the view function would normally not have control over. The following attributes can be provided optionally to either override some defaults to add_url_rule() or general behavior:

  • __name__: The name of a function is by default used as endpoint. If endpoint is provided explicitly this value is used. Additionally this will be prefixed with the name of the blueprint by default which cannot be customized from the function itself.
  • methods: If methods are not provided when the URL rule is added, Flask will look on the view function object itself if a methods attribute exists. If it does, it will pull the information for the methods from there.
  • provide_automatic_options: if this attribute is set Flask will either force enable or disable the automatic implementation of the HTTP OPTIONS response. This can be useful when working with decorators that want to customize the OPTIONS response on a per-view basis.
  • required_methods: if this attribute is set, Flask will always add these methods when registering a URL rule even if the methods were explicitly overridden in the route() call.

Full example:

def index():
    if request.method == 'OPTIONS':
        # custom options handling here
        ...
    return 'Hello World!'
index.provide_automatic_options = False
index.methods = ['GET', 'OPTIONS']

app.add_url_rule('/', index)
Changelog

New in version 0.8: The provide_automatic_options functionality was added.

Command Line Interface

class flask.cli.FlaskGroup(add_default_commands=True, create_app=None, add_version_option=True, load_dotenv=True, **extra)

Special subclass of the AppGroup group that supports loading more commands from the configured Flask app. Normally a developer does not have to interface with this class but there are some very advanced use cases for which it makes sense to create an instance of this.

For information as of why this is useful see Custom Scripts.

Parameters:
  • add_default_commands – if this is True then the default run and shell commands wil be added.
  • add_version_option – adds the --version option.
  • create_app – an optional callback that is passed the script info and returns the loaded app.
  • load_dotenv – Load the nearest .env and .flaskenv files to set environment variables. Will also change the working directory to the directory containing the first file found.
Changelog

Changed in version 1.0: If installed, python-dotenv will be used to load environment variables from .env and .flaskenv files.

get_command(ctx, name)

Given a context and a command name, this returns a Command object if it exists or returns None.

list_commands(ctx)

Returns a list of subcommand names in the order they should appear.

main(*args, **kwargs)

This is the way to invoke a script with all the bells and whistles as a command line application. This will always terminate the application after a call. If this is not wanted, SystemExit needs to be caught.

This method is also available by directly calling the instance of a Command.

New in version 3.0: Added the standalone_mode flag to control the standalone mode.

Changelog
Parameters:
  • args – the arguments that should be used for parsing. If not provided, sys.argv[1:] is used.
  • prog_name – the program name that should be used. By default the program name is constructed by taking the file name from sys.argv[0].
  • complete_var – the environment variable that controls the bash completion support. The default is "_<prog_name>_COMPLETE" with prog name in uppercase.
  • standalone_mode – the default behavior is to invoke the script in standalone mode. Click will then handle exceptions and convert them into error messages and the function will never return but shut down the interpreter. If this is set to False they will be propagated to the caller and the return value of this function is the return value of invoke().
  • extra – extra keyword arguments are forwarded to the context constructor. See Context for more information.
class flask.cli.AppGroup(name=None, commands=None, **attrs)

This works similar to a regular click Group but it changes the behavior of the command() decorator so that it automatically wraps the functions in with_appcontext().

Not to be confused with FlaskGroup.

command(*args, **kwargs)

This works exactly like the method of the same name on a regular click.Group but it wraps callbacks in with_appcontext() unless it’s disabled by passing with_appcontext=False.

group(*args, **kwargs)

This works exactly like the method of the same name on a regular click.Group but it defaults the group class to AppGroup.

class flask.cli.ScriptInfo(app_import_path=None, create_app=None)

Help object to deal with Flask applications. This is usually not necessary to interface with as it’s used internally in the dispatching to click. In future versions of Flask this object will most likely play a bigger role. Typically it’s created automatically by the FlaskGroup but you can also manually create it and pass it onwards as click object.

app_import_path = None

Optionally the import path for the Flask application.

create_app = None

Optionally a function that is passed the script info to create the instance of the application.

data = None

A dictionary with arbitrary data that can be associated with this script info.

load_app()

Loads the Flask app (if not yet loaded) and returns it. Calling this multiple times will just result in the already loaded app to be returned.

flask.cli.load_dotenv(path=None)

Load “dotenv” files in order of precedence to set environment variables.

If an env var is already set it is not overwritten, so earlier files in the list are preferred over later files.

Changes the current working directory to the location of the first file found, with the assumption that it is in the top level project directory and will be where the Python path should import local packages from.

This is a no-op if python-dotenv is not installed.

Parameters:path – Load the file at this location instead of searching.
Returns:True if a file was loaded.
Changelog

New in version 1.0.

flask.cli.with_appcontext(f)

Wraps a callback so that it’s guaranteed to be executed with the script’s application context. If callbacks are registered directly to the app.cli object then they are wrapped with this function by default unless it’s disabled.

flask.cli.pass_script_info(f)

Marks a function so that an instance of ScriptInfo is passed as first argument to the click callback.

flask.cli.run_command = <click.core.Command object>

Run a local development server.

This server is for development purposes only. It does not provide the stability, security, or performance of production WSGI servers.

The reloader and debugger are enabled by default if FLASK_ENV=development or FLASK_DEBUG=1.

flask.cli.shell_command = <click.core.Command object>

Runs an interactive Python shell in the context of a given Flask application. The application will populate the default namespace of this shell according to it’s configuration.

This is useful for executing small snippets of management code without having to manually configure the application.