Alternate PR to #3970

[mistercrunch]

No description provided.

[john-bodley]

LGTM

[mapto]

I also need to add a before_request for #7024 to get session information from cookies. However, this PR does not seem to solve the problem: the intended use as per the provided example does not work.

FLASK_APP_MUTATOR = lambda x: x.before_request = f

results in a SyntaxError: can't assign to lambda with Python 3.6.8. Currently, assignments inside lambda are not supported in Python, and this is apparently going to change in Python 3.8.

A functional alternative to the example would be:

def attach_handler(app):
    app.before_request = f

FLASK_APP_MUTATOR = lambda app: attach_handler(app)