django-after-response

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The function_queue variable is shared between all the threads on a process. If thread 1 adds something to the queue and then does some more processing and thread 2 comes along and finishes, even if it didn't add anything to the queue, it will process thread 1's function while thread 1 is still running.

You should replace function queue with a method that will pull the list from the threading.local and create it if it does not exist. Do not just put the variable in threading.local and leave it as a global variable. Then you'll just be sharing the thread local with all the other processes.

It should be something like this:

import threading
threadlocal = threading.local()    

def function_queue():
  queue = getattr(threadlocal, 'function_queue', None)
  if v is None:
    queue = []
    setattr(threadlocal, 'function_queue', queue)
  return queue

Would deque be a good idea? https://docs.python.org/3.6/library/collections.html#collections.deque

That might give you a speed increase. I think most people will only add a few functions so you'll see hardly any speed increase vs a list. It won't give you thread safety. You can use thread locals or you could add the fuctions to a property on the request object. That way even if people add functions from mutiple threads they'll be accociated with the request. I think hardly anyone would set callbacks from multiple threads though and you would have to start passing the request around to do that.