Command Line Interface

New in version 0.11.

One of the nice new features in Flask 0.11 is the built-in integration of the click command line interface. This enables a wide range of new features for the Flask ecosystem and your own applications.

Basic Usage

After installation of Flask you will now find a flask script installed into your virtualenv. If you don’t want to install Flask or you have a special use-case you can also use python -m flask to accomplish exactly the same.

The way this script works is by providing access to all the commands on your Flask application’s Flask.cli instance as well as some built-in commands that are always there. Flask extensions can also register more commands there if they desire so.

For the flask script to work, an application needs to be discovered. This is achieved by exporting the FLASK_APP environment variable. It can be either set to an import path or to a filename of a Python module that contains a Flask application.

In that imported file the name of the app needs to be called app or optionally be specified after a colon. For instance mymodule:application would tell it to use the application object in the mymodule.py file.

Given a hello.py file with the application in it named app this is how it can be run.

Environment variables (On Windows use set instead of export):

export FLASK_APP=hello
flask run

Or with a filename:

export FLASK_APP=/path/to/hello.py
flask run

Virtualenv Integration

If you are constantly working with a virtualenv you can also put the export FLASK_APP into your activate script by adding it to the bottom of the file. That way every time you activate your virtualenv you automatically also activate the correct application name.

Debug Flag

The flask script can also be instructed to enable the debug mode of the application automatically by exporting FLASK_DEBUG. If set to 1 debug is enabled or 0 disables it.

Or with a filename:

export FLASK_DEBUG=1

Running a Shell

To run an interactive Python shell you can use the shell command:

flask shell

This will start up an interactive Python shell, setup the correct application context and setup the local variables in the shell. This is done by invoking the Flask.make_shell_context() method of the application. By default you have access to your app and g.

Custom Commands

If you want to add more commands to the shell script you can do this easily. Flask uses click for the command interface which makes creating custom commands very easy. For instance if you want a shell command to initialize the database you can do this:

import click
from flask import Flask

app = Flask(__name__)

@app.cli.command()
def initdb():
    """Initialize the database."""
    click.echo('Init the db')

The command will then show up on the command line:

$ flask initdb
Init the db

Application Context

Most commands operate on the application so it makes a lot of sense if they have the application context setup. Because of this, if you register a callback on app.cli with the command() the callback will automatically be wrapped through cli.with_appcontext() which informs the cli system to ensure that an application context is set up. This behavior is not available if a command is added later with add_command() or through other means.

It can also be disabled by passing with_appcontext=False to the decorator:

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

Factory Functions

In case you are using factory functions to create your application (see Application Factories) you will discover that the flask command cannot work with them directly. Flask won’t be able to figure out how to instantiate your application properly by itself. Because of this reason the recommendation is to create a separate file that instantiates applications. This is not the only way to make this work. Another is the Custom Scripts support.

For instance if you have a factory function that creates an application from a filename you could make a separate file that creates such an application from an environment variable.

This could be a file named autoapp.py with these contents:

import os
from yourapplication import create_app
app = create_app(os.environ['YOURAPPLICATION_CONFIG'])

Once this has happened you can make the flask command automatically pick it up:

export YOURAPPLICATION_CONFIG=/path/to/config.cfg
export FLASK_APP=/path/to/autoapp.py

From this point onwards flask will find your application.

Custom Scripts

While the most common way is to use the flask command, you can also make your own “driver scripts”. Since Flask uses click for the scripts there is no reason you cannot hook these scripts into any click application. There is one big caveat and that is, that commands registered to Flask.cli will expect to be (indirectly at least) launched from a flask.cli.FlaskGroup click group. This is necessary so that the commands know which Flask application they have to work with.

To understand why you might want custom scripts you need to understand how click finds and executes the Flask application. If you use the flask script you specify the application to work with on the command line or environment variable as an import name. This is simple but it has some limitations. Primarily it does not work with application factory functions (see Application Factories).

With a custom script you don’t have this problem as you can fully customize how the application will be created. This is very useful if you write reusable applications that you want to ship to users and they should be presented with a custom management script.

To explain all of this, here is an example manage.py script that manages a hypothetical wiki application. We will go through the details afterwards:

import os
import click
from flask.cli import FlaskGroup

def create_wiki_app(info):
    from yourwiki import create_app
    return create_app(
        config=os.environ.get('WIKI_CONFIG', 'wikiconfig.py'))

@click.group(cls=FlaskGroup, create_app=create_wiki_app)
def cli():
    """This is a management script for the wiki application."""

if __name__ == '__main__':
    cli()

That’s a lot of code for not much, so let’s go through all parts step by step.

  1. First we import the click library as well as the click extensions from the flask.cli package. Primarily we are here interested in the FlaskGroup click group.

  2. The next thing we do is defining a function that is invoked with the script info object (ScriptInfo) from Flask and its purpose is to fully import and create the application. This can either directly import an application object or create it (see Application Factories). In this case we load the config from an environment variable.

  3. Next step is to create a FlaskGroup. In this case we just make an empty function with a help doc string that just does nothing and then pass the create_wiki_app function as a factory function.

    Whenever click now needs to operate on a Flask application it will call that function with the script info and ask for it to be created.

  4. All is rounded up by invoking the script.

CLI Plugins

Flask extensions can always patch the Flask.cli instance with more commands if they want. However there is a second way to add CLI plugins to Flask which is through setuptools. If you make a Python package that should export a Flask command line plugin you can ship a setup.py file that declares an entrypoint that points to a click command:

Example setup.py:

from setuptools import setup

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

Inside mypackage/comamnds.py you can then export a Click object:

import click

@click.command()
def cli():
    """This is an example command."""

Once that package is installed in the same virtualenv as Flask itself you can run flask my-command to invoke your command. This is useful to provide extra functionality that Flask itself cannot ship.