How to stop Flask from initialising twice in Debug Mode? [duplicate]

Question

When building a Flask service in Python and setting the debug mode on, the Flask service will initialise twice. When the initialisation loads caches and the like, this can take a while. Having to do this twice is annoying when in development (debug) mode. When debug is off, the Flask service only initialises once.

How to stop Flask from initialising twice in Debug Mode?

Tiliaceous

Answer 1

The simplest thing to do here would be to add use_reloader=False to your call to app.run - that is: app.run(debug=True, use_reloader=False)

Alternatively, you can check for the value of WERKZEUG_RUN_MAIN in the environment:

if os.environ.get("WERKZEUG_RUN_MAIN") == "true":
    # The reloader has already run - do what you want to do here

However, the condition is a bit more convoluted when you want the behavior to happen any time except in the loading process:

if not app.debug or os.environ.get("WERKZEUG_RUN_MAIN") == "true":
    # The app is not in debug mode or we are in the reloaded process

Serrato

How can you still use reloader but have certain things initialised only once? – Kamalakamaria

@osa - I added a brief explanation of an alternative that might let you use the reloader – Serrato

use_reloader=False worked perfectly for me. – Spool

This answer implies there is no solution, only a workaround? Disabling the reloader is not usually an acceptable trade off. – Cerussite

The solution is to either, "don't use the reloader" or "check for the WERKZEUG_RUN_MAIN environment variable before doing the expensive work. – Serrato

This does not work for me, because app.debug returns False even when Flask is in debug mode, i.e. app.run(host='0.'0.0.0', port=80, debug=True) – Tempestuous

It seems that the problem is that app.run should be the last command, and before you run app.run you cannot know the state of Flask. – Tempestuous

I found a corner case to this, it seems using FLASK_ENV=development does not set add.debug to true. In the end I'm using if not (app.debug or os.environ.get("FLASK_ENV") == "development") or os.environ.get("WERKZEUG_RUN_MAIN") == "true":. Good luck figure that one out in 6 months... – Scoundrel

Answer 2

You can use the before_first_request hook:

@app.before_first_request
def initialize():
    print "Called only once, when the first request comes in"

Unchurch

Thanks alex but if I am doing cache warming or pre populating stuff like a 'classifier' will this get called before the first request, making the first request slow or on startup of the flask service. – Tiliaceous

Downvote. When the app reloads, before_first_request will be called again after the first request that follows. – Kamalakamaria