Why does from flask.cli import main fail when I set FLASK_ENV?

Question

I have installed flask in a virtual environment as described here https://flask.palletsprojects.com/en/1.1.x/installation/#install-create-env

Create a directory and run python3 -m venv venv inside the directory to use virtual environment
Then source venv/bin/activate to activate the environment
Then pip install flask

Then I created a script like described here https://flask.palletsprojects.com/en/1.1.x/quickstart/

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello, World!'

Then I added export FLASK_APP=hello.py and ran python3 -m flask run which worked just fine

python3 -m flask run
 * Serving Flask app "hello.py"
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

But when I then change FLASK_ENV=development and run the same command I get an error

python3 -m flask run
 * Serving Flask app "hello.py" (lazy loading)
 * Environment: development
 * Debug mode: on
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with stat
/usr/local/opt/python/bin/python3.7: No module named flask

I unset the FLASK_ENV and ran the same command again and everything works as when I did this the first time.

TheSynt4x TheSynt4x · Accepted Answer · 2020-02-28T17:59:59

Well, one solution might be to not set any environment variables and run Flask with just python.

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello, World!'

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

and then you just activate the virtual environment by source env/bin/activate and then run python main.py.

You can also set the flask environment in your code by doing app.config['FLASK_ENV'] = 'development' but I wouldn't recommend it since if you're going to deploy your Flask app to platforms like Heroku or GCP, it's not going to work as intended.

Why does from flask.cli import main fail when I set FLASK_ENV?

2 Answers