[Python-Dev] PEP 492: async/await in Python; v3 (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Wed Apr 29 06:10:45 CEST 2015

On 2015-04-28 11:59 PM, Greg wrote:

On 29/04/2015 9:49 a.m., Guido van Rossum wrote:

c) 'yield from' only accept coroutine objects from generators decorated with 'types.coroutine'. That means that existing asyncio generator-based coroutines will happily yield from both coroutines and generators. But every generator-based coroutine must be decorated with asyncio.coroutine(). This is potentially a backwards incompatible change.

See below. I worry about backward compatibility. A lot. Are you saying that asycio-based code that doesn't use @coroutine will break in 3.5? That seems unavoidable if the goal is for 'await' to only work on generators that are intended to implement coroutines, and not on generators that are intended to implement iterators. Because there's no way to tell them apart without marking them in some way.

Not sure what you mean by "unavoidable".

Before the last revision of the PEP it was perfectly fine to use generators in 'yield from' in generator-based coroutines:

@asyncio.coroutine
def foo():
   yield from gen()

and yet you couldn't do the same with 'await' (as it has a special opcode instead of GET_ITER that can validate what you're awaiting).

With the new version of the PEP - 'yield from' in foo() would raise a TypeError. If we change it to a RuntimeWarning then we're safe in terms of backwards compatibility. I just want to see how exactly warnings will work (i.e. will they occur multiple times at the same 'yield from' expression, etc)

Yury

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