How to stop Flask from initialising twice in Debug Mode?

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

You can use the before_first_request hook:

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

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