Promise

[nilamo]

This all seems kinda crazy, so here's some more info.  Almost all code would be consumers of a promise, only a library would implement one.  The important part is that the code generating a promise should have a little bit of setup, but otherwise immediately return, and use the callbacks to signal when it's done doing it's work. 

Here's a sample use case, using a delayed timer with a callback, that gets called after that delay:

from promise import Promise
import threading
import time


def delayed(delay):
    def runner(resolved, failed):
        def thread():
            time.sleep(delay)
            resolved()
        threading.Thread(target=thread).start()

    return Promise(runner)


if __name__ == "__main__":
    def callback(*args):
        print("Inside callback")

    print("Before delayed callback")
    delayed(2.5).then(callback)
    print("After delayed callback")
    for _ in range(5):
        print("waiting...")
        time.sleep(1)

Output:
Before delayed callback
After delayed callback
waiting...
waiting...
waiting...
Inside callback
waiting...
waiting...