[Python-Dev] PEP 492 and types.coroutine

Ethan Furman ethan at stoneleaf.us
Sat May 2 20:14:04 CEST 2015

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.

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."

> '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


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