Chapter 2 of Miguel Grinberg’s Flask Web Development describes how to run a basic Flask app. Suppose you have a basic Flask app like below (from the book).
from flask import Flask app = Flask(__name__) @app.route('/') def index(): return '<h1>Hello World</h1>'
You can use the Python 3 built-in venv module to create a virtual environment like below.
$ python -m venv venv $ source venv/bin/activate $ pip install flask
Here is output of
pip freeze command. The
pip install command installs several Flask dependencies.
click==7.1.2 Flask==1.1.2 itsdangerous==1.1.0 Jinja2==2.11.2 MarkupSafe==1.1.1 Werkzeug==1.0.1
Then you can setup two environment variables and type
flask run to start the app. But what happens
when you start the command
flask run? I am trying to find out the answer and this article documents
$ export FLASK_APP=hello.py $ export FLASK_ENV=development $ flask run
When you run the command
pip install flask, the pip system installs a
flask python script file in the
venv/bin/ directory. You can think of
flask as a command and
run as an argument to the command.
run part is actually a sub-command. You can run
flask --help to find out other available
flask script file in
venv/bin/ directory only has a few lines as shown below. The script is
main() function in the flask.cli module.
import re import sys from flask.cli import main if __name__ == '__main__': sys.argv = re.sub(r'(-script\.pyw?|\.exe)?$', '', sys.argv) sys.exit(main())
Let’s open the
cli.py file in
venv\lib\python3.8\site-packages\flask directory. Line 965
main function. The
as_module argument is
False and the
cli.main method with two arguments
args=['run', ] and
cli.main is an instance of FlaskGroup class defined on Line 945.
def main(as_module=False): # TODO omit sys.argv once ... is fixed cli.main(args=sys.argv[1:], prog_name="python -m flask" if as_module else None)
Here things become complicated and I have not figured everything out. The FlaskGroup class
is defined on Line 462 and it has a main method defined on Line 567 (code shown below).
The method makes changes to some settings and calls the
main method in super class.
def main(self, *args, **kwargs): os.environ["FLASK_RUN_FROM_CLI"] = "true" if get_load_dotenv(self.load_dotenv): load_dotenv() obj = kwargs.get("obj") if obj is None: obj = ScriptInfo( create_app=self.create_app, set_debug_flag=self.set_debug_flag ) kwargs["obj"] = obj kwargs.setdefault("auto_envvar_prefix", "FLASK") return super(FlaskGroup, self).main(*args, **kwargs)
FlaskGroup is derived from
AppGroup class which is defined on Line 431. The
in turn is derived from
click.Group class defined in
core.py file in click package. The complete
class inheritance tree is shown below. The
main method in super class mentioned above is
defined all the way up in
FlaskGroup cli.py Line 462 AppGroup cli.py Line 431 click.Group click/core.py Line 1331 click.MultiCommand /core.py Line 1069 click.Command /core.py Line 832 click.BaseCommand /core.py Line 631
Let’s get back to the
flask/cli.py file and take a look at
AppGroup class. The
AppGroup class overrides two methods (decorators)
changes the behavior of the standard
command decorator so that it automatically
wraps the functions in
with_appcontext. You can see examples of how those two
decorators are used on the
click documentation page.
FlaskGroup class overrides
list_commands methods in addition
main method. The
has a section Custom Multi Commands and the example in this section also overrides
those two methods.
run command is defined on Line 828 and the function is
run becomes part of flask built in commands loaded by default.
The code which loads the built in commands is in the
run_command function look like this. It calls the
run_simple function in
werkzeug module and starts the development server. The
is also interesting, and it loads the app based on the environment variable
settings. I will discuss it in a later article.
@click.command("run", short_help="Run a development server.") @click.option("--host", "-h", default="127.0.0.1", ...") ... @pass_script_info def run_command(info, host, port, ...): """Run a local development server.""" ...... show_server_banner(get_env(), debug, info.app_import_path, eager_loading) app = DispatchingApp(info.load_app, use_eager_loading=eager_loading) from werkzeug.serving import run_simple run_simple(host, port, app, ......)
routes commands are defined on Line 864 and Line 899 of
The Flask documentation has a page Command Line Interface which has very good info on Flask CLI.