What does the “yield from” syntax do in asyncio an

Question

From the perspective of someone who has written asyncio code but is looking to better understand the inner workings, what is yield from, await and how are those useful for allowing asynchronous code?

There is one highly upvoted question asking about the uses of the yield from syntax and one explaining async and await, but both go in depth about different topics and are not really a concise explanation of the underlying code and how it fits in with asyncio.

Answer 1

Short answer:

yield from is an old way to wait for asyncio's coroutine.

await is an modern way to wait for asyncio's coroutine.

Detailed answer:

Python has generators - special kind of functions that produces a sequence of results instead of a single value. Starting with Python 3.3 yield from expression was added. It allows one generator to delegate part of its operations to another generator.

Starting with Python 3.4 asyncio module was added to standard library. It allow us to write clear and understandable asynchronous code. While technically asyncio's coroutines could be implemented different ways, in asyncio they were implemented using generators (you can watch for excellent video where shown how generators can be used to implement coroutines). @asyncio.coroutine was a way to make coroutine from generator and yield from was a way to await for coroutine - just details of implementation.

That's how happened that yield from started to be used for two "different things".

Starting with Python 3.5 (see PEP 492) coroutines got new syntax. Now you can define coroutine with async def and await for it using await expression. It's not only shorter to write, but also makes clearer to understand that we work with asyncio's coroutines.

If you're using Python 3.5+ you can forget about using yield from for asyncio's coroutines and use await for it.