flask run is enough because real command is : FLASK_APP=app.py flask run

I have provided application login in app.py in Path environment using export FLASK_APP=app.py

The app.py it is pointing at

FLASK_APP=app.py only means something if there is an app.py with a Flask instance in it. The smallest version:

from flask import Flask

app = Flask(__name__)


@app.route("/")
def index():
    return "Hello, world!"


if __name__ == "__main__":
    app.run(debug=True)

Flask(__name__) tells Flask where the application lives so it can resolve templates and static files relative to that module. @app.route("/") binds a URL path to the function below it, and whatever that function returns becomes the response body.

Newer Flask versions let you skip the environment variable and name the module on the command line instead:

flask --app app run --debug

--app app points at app.py and --debug does the same job as FLASK_DEBUG=1. The app.run(debug=True) block is a third way in — it runs when you execute python app.py directly, and it is skipped when you start the app through the flask command, because the module is imported rather than run as __main__.

we need to enable debug mode when we’re working on our project and server is watching for changes.

FLASK_APP=app.py FLASK_DEBUG=1 flask run

To work with python environment:

lsvirtualenv
mkvirtualenv first-flask-app
workon first-flask-app
pip install flask
deactivate

Keep the environment per project, not per machine. pip install flask inside an activated environment installs into that environment only, so two Flask projects can sit on different versions without interfering. That is the whole reason the workon step comes before the pip install step above.

install dependencies

Find dependencies of a Project:

pip list
pip freeze

How to find a current shell is bash or zsh

$ echo $0
$ echo $SHELL

echo $0 is more accurate and if you type bash in zsh shell

To change prompt to include a new line

PS1="$PS1\n"

Uninstall existing versions

pip uninstall werkzeug Flask-Login

Reinstall all dependencies

pip freeze > requirements.txt
pip install -r requirements.txt

Pin what you install

pip freeze writes exact versions, which is what you want committed. An unpinned flask line installs whatever happens to be current the next time someone sets the project up, and that is how a project that worked last month stops working. Regenerate the file with the environment active, and rebuild from it with pip install -r requirements.txt.

Turn debug mode off in production

Debug mode belongs on your machine only. When an unhandled exception occurs it serves the Werkzeug traceback page, which includes an interactive console that can execute Python on the server. It is PIN-protected, but it is still a remote code execution path that should never be reachable from the internet. Debug mode also reloads the process on every file change, which you do not want in a running service.

The development server itself is not meant for production either. Deploy behind a WSGI server such as Gunicorn or uWSGI — or Waitress on Windows — and leave debug off.

Flask and Virtual Environments in Python