Python Coroutines!?!?

Python supports generators which allow you to .send() and recieve (via next(...)) values. They are kind of like channels since they don't block until you send or recieve.

In the code below, we use callbacks (called aperiodically in a separate thread) to send values to our channel. Simultaneously, we try to consume those values, which should be allowed because generators and our coroutine are non-blocking.

import time, threading

def channel(x="Hello"):
    while True:
        x = yield x

def make_coroutine(callback):
    def coroutine():
        callback()
        threading.Timer(1, coroutine).start()
    return coroutine

def make_callback():
    chan = channel()
    # prime the channel
    next(chan)

    def callback():
        print("Calling!")
        chan.send("Hello world!")
    
    return chan, callback

def main():
    chan, callback = make_callback()
    coroutine = make_coroutine(callback)
    coroutine()
    for i in chan:
        if i is not None:
            print(i)

if __name__ == "__main__":
    main()

And yet, this approach doesn't work as expected! One of three things happens:

1. ValueError: generator already executing
2. only None values are output from the channel

Why doesn't this work? email me if you have an answer.