Command Line Interface

Installing Flask installs the flask script, a Click command line interface, in your virtualenv. Executed from the terminal, this script gives access to built-in, extension, and application-defined commands. The --help option will give more information about any commands and options.

Application Discovery

The flask command is installed by Flask, not your application; it must be told where to find your application in order to use it. The FLASK_APP environment variable is used to specify how to load the application.

Unix Bash (Linux, Mac, etc.):

$ export FLASK_APP=hello
$ flask run

Windows CMD:

> set FLASK_APP=hello
> flask run

Windows PowerShell:

> $env:FLASK_APP = "hello"
> flask run

While FLASK_APP supports a variety of options for specifying your application, most use cases should be simple. Here are the typical values:

(nothing)
The file wsgi.py is imported, automatically detecting an app (app). This provides an easy way to create an app from a factory with extra arguments.
FLASK_APP=hello
The name is imported, automatically detecting an app (app) or factory (create_app).

FLASK_APP has three parts: an optional path that sets the current working directory, a Python file or dotted import path, and an optional variable name of the instance or factory. If the name is a factory, it can optionally be followed by arguments in parentheses. The following values demonstrate these parts:

FLASK_APP=src/hello
Sets the current working directory to src then imports hello.
FLASK_APP=hello.web
Imports the path hello.web.
FLASK_APP=hello:app2
Uses the app2 Flask instance in hello.
FLASK_APP="hello:create_app('dev')"
The create_app factory in hello is called with the string 'dev' as the argument.

If FLASK_APP is not set, the command will look for a file called wsgi.py or app.py and try to detect an application instance or factory.

Within the given import, the command looks for an application instance named app or application, then any application instance. If no instance is found, the command looks for a factory function named create_app or make_app that returns an instance.

When calling an application factory, if the factory takes an argument named info, then the ScriptInfo instance is passed as a keyword argument. If parentheses follow the factory name, their contents are parsed as Python literals and passes as arguments to the function. This means that strings must still be in quotes.

Run the Development Server

The run command will start the development server. It replaces the Flask.run() method in most cases.

$ flask run
 * Serving Flask app "hello"
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

Warning

Do not use this command to run your application in production. Only use the development server during development. The development server is provided for convenience, but is not designed to be particularly secure, stable, or efficient. See Deployment Options for how to run in production.

Open a Shell

To explore the data in your application, you can start an interactive Python shell with the shell command. An application context will be active, and the app instance will be imported.

$ flask shell
Python 3.6.2 (default, Jul 20 2017, 03:52:27)
[GCC 7.1.1 20170630] on linux
App: example
Instance: /home/user/Projects/hello/instance
>>>

Use shell_context_processor() to add other automatic imports.

Environments

New in version 1.0.

Changelog

The environment in which the Flask app runs is set by the FLASK_ENV environment variable. If not set it defaults to production. The other recognized environment is development. Flask and extensions may choose to enable behaviors based on the environment.

If the env is set to development, the flask command will enable debug mode and flask run will enable the interactive debugger and reloader.

$ FLASK_ENV=development flask run
 * Serving Flask app "hello"
 * Environment: development
 * Debug mode: on
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with inotify reloader
 * Debugger is active!
 * Debugger PIN: 223-456-919

Debug Mode

Debug mode will be enabled when FLASK_ENV is development, as described above. If you want to control debug mode separately, use FLASK_DEBUG. The value 1 enables it, 0 disables it.

Environment Variables From dotenv

Rather than setting FLASK_APP each time you open a new terminal, you can use Flask’s dotenv support to set environment variables automatically.

If python-dotenv is installed, running the flask command will set environment variables defined in the files .env and .flaskenv. This can be used to avoid having to set FLASK_APP manually every time you open a new terminal, and to set configuration using environment variables similar to how some deployment services work.

Variables set on the command line are used over those set in .env, which are used over those set in .flaskenv. .flaskenv should be used for public variables, such as FLASK_APP, while .env should not be committed to your repository so that it can set private variables.

Directories are scanned upwards from the directory you call flask from to locate the files. The current working directory will be set to the location of the file, with the assumption that that is the top level project directory.

The files are only loaded by the flask command or calling run(). If you would like to load these files when running in production, you should call load_dotenv() manually.

Setting Command Options

Click is configured to load default values for command options from environment variables. The variables use the pattern FLASK_COMMAND_OPTION. For example, to set the port for the run command, instead of flask run --port 8000:

export FLASK_RUN_PORT=8000
flask run
 * Running on http://127.0.0.1:8000/

These can be added to the .flaskenv file just like FLASK_APP to control default command options.

Disable dotenv

The flask command will show a message if it detects dotenv files but python-dotenv is not installed.

flask run
 * Tip: There are .env files present. Do "pip install python-dotenv" to use them.

You can tell Flask not to load dotenv files even when python-dotenv is installed by setting the FLASK_SKIP_DOTENV environment variable. This can be useful if you want to load them manually, or if you’re using a project runner that loads them already. Keep in mind that the environment variables must be set before the app loads or it won’t configure as expected.

export FLASK_SKIP_DOTENV=1
flask run

Environment Variables From virtualenv

If you do not want to install dotenv support, you can still set environment variables by adding them to the end of the virtualenv’s activate script. Activating the virtualenv will set the variables.

Unix Bash, venv/bin/activate:

export FLASK_APP=hello

Windows CMD, venv\Scripts\activate.bat:

set FLASK_APP=hello

It is preferred to use dotenv support over this, since .flaskenv can be committed to the repository so that it works automatically wherever the project is checked out.

Custom Commands

The flask command is implemented using Click. See that project’s documentation for full information about writing commands.

This example adds the command create_user that takes the argument name.

import click
from flask import Flask

app = Flask(__name__)

@app.cli.command()
@click.argument('name')
def create_user(name):
    ...
flask create_user admin

This example adds the same command, but as user create, a command in a group. This is useful if you want to organize multiple related commands.

import click
from flask import Flask
from flask.cli import AppGroup

app = Flask(__name__)
user_cli = AppGroup('user')

@user_cli.command('create')
@click.argument('name')
def create_user(name):
    ...

app.cli.add_command(user_cli)
flask user create demo

See Testing CLI Commands for an overview of how to test your custom commands.

Application Context

Commands added using the Flask app’s cli command() decorator will be executed with an application context pushed, so your command and extensions have access to the app and its configuration. If you create a command using the Click command() decorator instead of the Flask decorator, you can use with_appcontext() to get the same behavior.

import click
from flask.cli import with_appcontext

@click.command
@with_appcontext
def do_work():
    ...

app.cli.add_command(do_work)

If you’re sure a command doesn’t need the context, you can disable it:

@app.cli.command(with_appcontext=False)
def do_work():
    ...

Plugins

Flask will automatically load commands specified in the flask.commands entry point. This is useful for extensions that want to add commands when they are installed. Entry points are specified in setup.py

from setuptools import setup

setup(
    name='flask-my-extension',
    ...,
    entry_points={
        'flask.commands': [
            'my-command=flask_my_extension.commands:cli'
        ],
    },
)

Inside flask_my_extension/commands.py you can then export a Click object:

import click

@click.command()
def cli():
    ...

Once that package is installed in the same virtualenv as your Flask project, you can run flask my-command to invoke the command.

Custom Scripts

When you are using the app factory pattern, it may be more convenient to define your own Click script. Instead of using FLASK_APP and letting Flask load your application, you can create your own Click object and export it as a console script entry point.

Create an instance of FlaskGroup and pass it the factory:

import click
from flask import Flask
from flask.cli import FlaskGroup

def create_app():
    app = Flask('wiki')
    # other setup
    return app

@click.group(cls=FlaskGroup, create_app=create_app)
def cli():
    """Management script for the Wiki application."""

Define the entry point in setup.py:

from setuptools import setup

setup(
    name='flask-my-extension',
    ...,
    entry_points={
        'console_scripts': [
            'wiki=wiki:cli'
        ],
    },
)

Install the application in the virtualenv in editable mode and the custom script is available. Note that you don’t need to set FLASK_APP.

$ pip install -e .
$ wiki run

Errors in Custom Scripts

When using a custom script, if you introduce an error in your module-level code, the reloader will fail because it can no longer load the entry point.

The flask command, being separate from your code, does not have this issue and is recommended in most cases.

PyCharm Integration

Prior to PyCharm 2018.1, the Flask CLI features weren’t yet fully integrated into PyCharm. We have to do a few tweaks to get them working smoothly. These instructions should be similar for any other IDE you might want to use.

In PyCharm, with your project open, click on Run from the menu bar and go to Edit Configurations. You’ll be greeted by a screen similar to this:

screenshot of pycharm's run configuration settings

There’s quite a few options to change, but once we’ve done it for one command, we can easily copy the entire configuration and make a single tweak to give us access to other commands, including any custom ones you may implement yourself.

Click the + (Add New Configuration) button and select Python. Give the configuration a good descriptive name such as “Run Flask Server”. For the flask run command, check “Single instance only” since you can’t run the server more than once at the same time.

Select Module name from the dropdown (A) then input flask.

The Parameters field (B) is set to the CLI command to execute (with any arguments). In this example we use run, which will run the development server.

You can skip this next step if you’re using Environment Variables From dotenv. We need to add an environment variable (C) to identify our application. Click on the browse button and add an entry with FLASK_APP on the left and the Python import or file on the right (hello for example).

Next we need to set the working directory (D) to be the folder where our application resides.

If you have installed your project as a package in your virtualenv, you may untick the PYTHONPATH options (E). This will more accurately match how you deploy the app later.

Click Apply to save the configuration, or OK to save and close the window. Select the configuration in the main PyCharm window and click the play button next to it to run the server.

Now that we have a configuration which runs flask run from within PyCharm, we can copy that configuration and alter the Script argument to run a different CLI command, e.g. flask shell.