[Python-Dev] Thoughts on "contexts". PEPs 550, 555, 567, 568

[Guido van Rossum]

The current status of PEP 555 is "Withdrawn". I have no interest in
considering it any more, so if you'd rather see a decision from me I'll be
happy to change it to "Rejected".

On Tue, Jan 9, 2018 at 10:29 PM, Koos Zevenhoven <k7hoven at gmail.com> wrote:

> On Jan 10, 2018 07:17, "Yury Selivanov" <yselivanov.ml at gmail.com> wrote:
>
> Wasn't PEP 555 rejected by Guido? What's the point of this post?
>
>
> I sure hope there is a point. I don't think mentioning PEP 555 in the
> discussions should hurt.
>
> A typo in my post btw: should be "PEP 567 (+568 ?)" in the second
> paragraph of course.
>
> -- Koos (mobile)
>
>
> Yury
>
> On Wed, Jan 10, 2018 at 4:08 AM Koos Zevenhoven <k7hoven at gmail.com> wrote:
>
>> Hi all,
>>
>> I feel like I should write some thoughts regarding the "context"
>> discussion, related to the various PEPs.
>>
>> I like PEP 567 (+ 567 ?) better than PEP 550. However, besides providing
>> cvar.set(), I'm not really sure about the gain compared to PEP 555 (which
>> could easily have e.g. a dict-like interface to the context). I'm still not
>> a big fan of "get"/"set" here, but the idea was indeed to provide those on
>> top of a PEP 555 type thing too.
>>
>> "Tokens" in PEP 567, seems to resemble assignment context managers in PEP
>> 555. However, they feel a bit messy to me, because they make it look like
>> one could just set a variable and then revert the change at any point in
>> time after that.
>>
>> PEP 555 is in fact a simplification of my previous sketch that had a
>> .set(..) in it, but was somewhat different from PEP 550. The idea was to
>> always explicitly define the scope of contextvar values. A context manager
>> / with statement determined the scope of .set(..) operations inside the
>> with statement:
>>
>> # Version A:
>> cvar.set(1)
>> with context_scope():
>>     cvar.set(2)
>>
>>     assert cvar.get() == 2
>>
>> assert cvar.get() == 1
>>
>> Then I added the ability to define scopes for different variables
>> separately:
>>
>> # Version B
>> cvar1.set(1)
>> cvar2.set(2)
>> with context_scope(cvar1):
>>     cvar1.set(11)
>>     cvar2.set(22)
>>
>> assert cvar1.get() == 1
>> assert cvar2.get() == 22
>>
>>
>> However, in practice, most libraries would wrap __enter__, set and
>> __exit__ into another context manager. So maybe one might want to allow
>> something like
>>
>> # Version C:
>> assert cvar.get() == something
>> with context_scope(cvar, 2):
>>     assert cvar.get() == 2
>>
>> assert cvar.get() == something
>>
>>
>> But this then led to combining "__enter__" and ".set(..)" into
>> Assignment.__enter__ -- and "__exit__" into Assignment.__exit__ like this:
>>
>> # PEP 555 draft version:
>> assert cvar.value == something
>> with cvar.assign(1):
>>     assert cvar.value == 1
>>
>> assert cvar.value == something
>>
>>
>> Anyway, given the schedule, I'm not really sure about the best thing to
>> do here. In principle, something like in versions A, B and C above could be
>> done (I hope the proposal was roughly self-explanatory based on earlier
>> discussions). However, at this point, I'd probably need a lot of help to
>> make that happen for 3.7.
>>
>> -- Koos
>>
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>
>
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-- 
--Guido van Rossum (python.org/~guido)
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