What's the right approach for calling functions after a flask app is run?

Question

I'm a little confused about how to do something that I thought would be quite simple. I have a simple app written using Flask. It looks something like this:

from flask import Flask  app = Flask(__name__)  def _run_on_start(a_string):     print "doing something important with %s" % a_string  @app.route('/') def root():     return 'hello world'  if __name__ == "__main__":     if len(sys.argv) < 2:         raise Exception("Must provide domain for application execution.")     else:         DOM = sys.argv[1]         _run_on_start("%s" % DOM)         app.run(debug=True) 

What I'm finding is that my terminal is outputting the print statements in _run_on_start but non of the other usual Flask app debug code. If I remove the call before app.run, the output is normal. Further I'm finding the output of _run_on_start to be repeated twice on startup, though I don't know if it's some weird output or the function is actually being called twice.

I'm assuming this is not the right way to add a function call before you call app.run. I looked in the Flask docs and found mentions of various decorators one can use, which allow you to execute a function before/after certain requests but I want to execute the call when the app server is run.

Further, I realise that if I call this module from another module, i.e., not when __name__ != "__main__" my I won't get my call to _run_on_start.

What's the right approach here? In both cases when I'm starting from the CL and from another module?

Answer 1

Probably you were looking for Flask.before_first_request decorator, as in:

@app.before_first_request def _run_on_start(a_string):     print "doing something important with %s" % a_string 

Answer 2

The duplicate output from your function can be explained by the reloader. The first thing it does is start the main function in a new thread so it can monitor the source files and restart the thread when they change. Disable this with the use_reloader=False option.

If you want to be able to run your function when starting the server from a different module, wrap it in a function, and call that function from the other module:

def run_server(dom):         _run_on_start("%s" % dom)         app.run(debug=True, use_reloader=False)  if __name__ == '__main__':     if len(sys.argv) < 2:         raise Exception("Must provide domain for application execution.")     else:         DOM = sys.argv[1]         run_server(DOM) 

The "right approach" depends on what you're actually trying to accomplish here. The built-in server is meant for running your application in a local testing environment before deploying it to a production server, so the problem of starting it from a different module doesn't make much sense on its own.