[Python-Dev] PEP 550 v4 (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Thu Sep 7 02:41:43 EDT 2017

On Wed, Sep 6, 2017 at 11:26 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

Guido van Rossum wrote:

This feels like a very abstract argument. I have a feeling that context state propagating out of a call is used relatively rarely -- it must work for cases where you refactor something that changes context inline into a utility function (e.g. decimal.setcontext()), but I just can't think of a realistic example where coroutines (either of the yield-from variety or of the async/def form) would be used for such a utility function. Yuri has already found one himself, the aenter and aexit methods of an async context manager.

aenter is not a generator and there's no 'yield from' there. Coroutines (within an async task) leak state just like regular functions (within a thread).

Your argument is to allow generators to leak context changes (right?). AFAIK we don't use generators to implement enter or aenter (generators decorated with @types.coroutine or @asyncio.coroutine are coroutines, according to PEP 492). So this is irrelevant.

A utility function that sets context state but also makes a network call just sounds like asking for trouble! I'm coming from the other direction. It seems to me that it's not very useful to allow with-statements to be skipped in certain very restricted circumstances.

Can you clarify what do you mean by "with-statements to be skipped"? This language is not used in PEP 550 or in Python documentation. I honestly don't understand what it means.

The only situation in which you will be able to take advantage of this is if the context change is being made in a generator or coroutine, and it is to apply to the whole body of that generator or coroutine. If you're in an ordinary function, you'll still have to use a context manager. If you only want the change to apply to part of the body, you'll still have to use a context manager. It would be simpler to just tell people to always use a context manager, wouldn't it?

Yes, PEP 550 wants people to always use a context managers! Which will work as you expect them to work for coroutines, generators, and regular functions. At this point I suspect you have some wrong idea about some specification detail of PEP 550. I understand what Koos is talking about, but I really don't follow you. Using the "with-statements to be skipped" language is very confusing and doesn't help to understand you.

Yury

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