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 --app option is used to specify how to load the application.

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

(nothing)

The name “app” or “wsgi” is imported (as a “.py” file, or package), automatically detecting an app (app or application) or factory (create_app or make_app).

--app hello

The given name is imported, automatically detecting an app (app or application) or factory (create_app or make_app).

--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:

--app src/hello

Sets the current working directory to src then imports hello.

--app hello.web

Imports the path hello.web.

--app hello:app2

Uses the app2 Flask instance in hello.

--app 'hello:create_app("dev")'

The create_app factory in hello is called with the string 'dev' as the argument.

If --app is not set, the command will try to import “app” or “wsgi” (as a “.py” file, or package) 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.

If parentheses follow the factory name, their contents are parsed as Python literals and passed as arguments and keyword 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 --app hello 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 Deploying to Production for how to run in production.

If another program is already using port 5000, you’ll see OSError: [Errno 98] or OSError: [WinError 10013] when the server tries to start. See Address already in use for how to handle that.

Debug Mode

In debug mode, the flask run command will enable the interactive debugger and the reloader by default, and make errors easier to see and debug. To enable debug mode, use the --debug option.

$ flask --app hello --debug run
 * Serving Flask app "hello"
 * 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

Watch and Ignore Files with the Reloader

When using debug mode, the reloader will trigger whenever your Python code or imported modules change. The reloader can watch additional files with the --extra-files option. Multiple paths are separated with :, or ; on Windows.

$ flask run --extra-files file1:dirA/file2:dirB/
 * Running on http://127.0.0.1:8000/
 * Detected change in '/path/to/file1', reloading

The reloader can also ignore files using fnmatch patterns with the --exclude-patterns option. Multiple patterns are separated with :, or ; on Windows.

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 3.10.0 (default, Oct 27 2021, 06:59:51) [GCC 11.1.0] on linux
App: example [production]
Instance: /home/david/Projects/pallets/flask/instance
>>>

Use shell_context_processor() to add other automatic imports.

Environment Variables From dotenv

The flask command supports setting any option for any command with environment variables. The variables are named like FLASK_OPTION or FLASK_COMMAND_OPTION, for example FLASK_APP or FLASK_RUN_PORT.

Rather than passing options every time you run a command, or environment variables every 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. You can also specify an extra file to load with the --env-file option. Dotenv files can be used to avoid having to set --app or FLASK_APP manually, 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 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

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("create-user")
@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 Running Commands with the CLI Runner for an overview of how to test your custom commands.

Registering Commands with Blueprints

If your application uses blueprints, you can optionally register CLI commands directly onto them. When your blueprint is registered onto your application, the associated commands will be available to the flask command. By default, those commands will be nested in a group matching the name of the blueprint.

from flask import Blueprint

bp = Blueprint('students', __name__)

@bp.cli.command('create')
@click.argument('name')
def create(name):
    ...

app.register_blueprint(bp)

$ flask students create alice

You can alter the group name by specifying the cli_group parameter when creating the Blueprint object, or later with app.register_blueprint(bp, cli_group='...'). The following are equivalent:

bp = Blueprint('students', __name__, cli_group='other')
# or
app.register_blueprint(bp, cli_group='other')

$ flask other create alice

Specifying cli_group=None will remove the nesting and merge the commands directly to the application’s level:

bp = Blueprint('students', __name__, cli_group=None)
# or
app.register_blueprint(bp, cli_group=None)

$ flask create alice

Application Context

Commands added using the Flask app’s cli or FlaskGroup command() decorator will be executed with an application context pushed, so your custom commands and parameters have access to the app and its configuration. The with_appcontext() decorator can be used to get the same behavior, but is not needed in most cases.

import click
from flask.cli import with_appcontext

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

app.cli.add_command(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 --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 --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

PyCharm Professional provides a special Flask run configuration to run the development server. For the Community Edition, and for other commands besides run, you need to create a custom run configuration. These instructions should be similar for any other IDE you use.

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

Screenshot of PyCharm run configuration.

Once you create a configuration for the flask run, you can copy and change it to call any other command.

Click the + (Add New Configuration) button and select Python. Give the configuration a name such as “flask run”.

Click the Script path dropdown and change it to Module name, then input flask.

The Parameters field is set to the CLI command to execute along with any arguments. This example uses --app hello --debug run, which will run the development server in debug mode. --app hello should be the import or file with your Flask app.

If you installed your project as a package in your virtualenv, you may uncheck the PYTHONPATH options. This will more accurately match how you deploy later.

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

Now that you have a configuration for flask run, you can copy that configuration and change the Parameters argument to run a different CLI command.