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

Ethan Furman ethan at stoneleaf.us
Sat May 2 23:31:37 CEST 2015

On 05/02, Yury Selivanov wrote:

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:

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.

Your "fix" doesn't fix it. I can decorate a non-coroutine generator with type.coroutine and it will still be broken and a bug in my code.

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?

I wouldn't, that would be a bug; but decorating a wrong type of generator is still a bug, and type.coroutine has not fixed that bug.

It's worse than mandatory typing because it can't even check that what I have declared is true.

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

I have read it, more than once. If you lift the (brand-new) requirement that a generator must be decorated, then type.coroutine becomes optional (no more useful, just optional). It is not a current requirement that coroutine generators be decorated.

-- Ethan

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