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

[Yury Selivanov]

Wasn't PEP 555 rejected by Guido? What's the point of this post?

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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