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