How to call a function when Apache terminates a Flask process?

I have a Flask app running behind Apache HTTPD. Apache is configured to have multiple child processes.

The Flask app creates a file on the server with the file's name equal to its process ID. The code looks something like this:

import os

@app.before_first_request
def before_first_request():
    filename = os.getpid()
    with open(filename, 'w') as file:
        file.write('Hello')

When the child process is killed/ended/terminated I would like the Flask app to remove this file.

It's not terribly important that removal of a file happens, as these files won't take up much space, so if bizarre errors occur I don't need to handle them. But for normal workflow, I would like to have some cleanup when Apache shuts down the Flask process.

Any idea on the best way to do this?

Solution

The best way to add cleanup functionality before graceful termination of a server-controlled Python process (such as a Flask app running in Apache WSGI context, or even better, in Gunicorn behind Apache) is using the atexit exit handler.

Elaborating on your original example, here's the addition of the exit handler doing the cleanup of .pid file:

import atexit
import os

filename = '{}.pid'.format(os.getpid())

@app.before_first_request
def before_first_request():
    with open(filename, 'w') as file:
        file.write('Hello')

def cleanup():
    try:
        os.remove(filename)
    except Exception:
        pass

atexit.register(cleanup)