[Python-Dev] PEP 550 v4

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.


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