Sequential method callbacks. In Appengine this would allow a deferred task to defer another task, and this task would defer another etc. so that tasks are completed sequentially

callback.py

#!/usr/bin/env python
from time import sleep


def simulate_defer(_callable, *args, **kwargs):
    """mocking Appengine's deferred.defer"""
    sleep(2)
    _callable(*args, **kwargs)


def a(callables):
    print('a')
    callback(callables)


def b(callables):
    print('b')
    callback(callables)


def c(callables):
    print('c')
    callback(callables)


def callback(callables):
    """
    :param callables: a list of callables
    :return:
    """
    if callables:
        _callable = callables.pop(0)
        simulate_defer(_callable, callables)


if __name__ == "__main__":
    # run a, then b and then c in that order.  Useful when functions are run synchronously for example
    # if they are being delegated to a task queue which has a limited timeout per task
    a([b, c])