[Python-Dev] PEP 492 and types.coroutine (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Sat May 2 22:10:12 CEST 2015

On 2015-05-02 2:14 PM, Ethan Furman wrote:

On 05/02, Yury Selivanov wrote:

On 2015-05-02 1:04 PM, Ethan Furman wrote:

If I'm understanding this correctly, type.coroutine's only purpose is to add a flag to a generator object so that await will accept it.

This raises the question of why can't await simply accept a generator object? There is no code change to the gen obj itself, there is no behavior change in the gen obj, it's the exact same byte code, only a flag is different. Because we don't want 'await' to accept random generators. It can't do anything meaningful with them, in a world where all asyncio code is written with new syntax, passing generator to 'await' is just a bug. And yet in current asyncio code, random generators can be accepted, and not even the current asyncio.coroutine wrapper can gaurantee that the generator is a coroutine in fact.

This is a flaw in the current Python that we want to fix.

For that matter, even the new types.coroutine cannot gaurantee that the returned object is a coroutine and not a generator -- so basically it's just there to tell the compiler, "yeah, I really know what I'm doing, shut up and do what I asked."

Well, why would you use it on some generator that is not a generator-based coroutine?

'types.coroutine' is something that we need to ease transition to the new syntax. This doesn't make sense -- either the existing generators are correctly returning coroutines, in which case the decorator adds nothing, or they are returning non-coroutines, in which case the decorator adds nothing. So either way, nothing has been added besides a mandatory boiler-plate requirement.

It's not nothing; it's backwards compatibility. Please read https://www.python.org/dev/peps/pep-0492/#await-expression

@types.coroutine marks generator function that its generator is awaitable.

Yury

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