14.5 AsyncIO

AsyncIO combines the concept of futures and an event loop with coroutines. The result is helpful for writing responsive applications that don’t seem to waste time waiting for input.

For the purposes of working with Python’s async features, a coroutine is a function that is waiting for an event, and also can provide events to other coroutines. In Python, we implement coroutines using async def. A function with async must work in the context of an event loop, which switches control of the thread among the coroutines waiting for events. We’ll see a few Python constructs using await expressions to show where the event loop can switch to another waiting async function.

It’s crucial to recognize that async operations are interleaved, and not — generally — parallel. At most one coroutine is in control and processing, and all the others are waiting for an event. The idea of interleaving is described as cooperative...