[Python-Dev] PEP 567 -- Context Variables

[Yury Selivanov]

On Tue, Dec 12, 2017 at 9:55 PM, Guido van Rossum <guido at python.org> wrote:
> On Tue, Dec 12, 2017 at 5:35 PM, Yury Selivanov <yselivanov.ml at gmail.com>
> wrote:
>>
>> On Tue, Dec 12, 2017 at 6:49 PM, Victor Stinner
>> <victor.stinner at gmail.com> wrote:
>> > I like the overall idea and I prefer this PEP over PEP 550 since it's
>> > shorter and easier to read :-)
>> >
>> > Question: Is there an API to list all context variables?
>>
>> Context implements abc.Mapping, so 'get_context().keys()' will give
>> you a list of all ContextVars in the current context.
>
>
> This was hinted at in the PEP, but maybe an explicit example would be nice.

Sure.

>
>>
>> > Each get_context() call returns a new Context object. It may be worth
>> > to mention it. I understand why, but it's surprising that "assert
>> > get_context() is not get_context()" fails. Maybe it's a naming issue?
>> > Maybe rename it to contextvars.context()?
>>
>> I think the name is fine.  While get_context() will return a new instance
>> every time you call it, those instances will have the same context
>> variables/values in them, so I don't think it's a problem.
>
>
> I'm fine with this, but perhaps == should be supported so that those two are
> guaranteed to be considered equal? (Otherwise an awkward idiom to compare
> contexts using expensive dict() copies would be needed to properly compare
> two contexts for equality.)

I've no problem with implementing 'Context.__eq__'.  I think
abc.Mapping also implements it.

>
>>
>> > At the first read, I understood that that ctx.run() creates a new
>> > temporary context which is removed once ctx.run() returns.
>> >
>> > Now I understand that context variable values are restored to their
>> > previous values once run() completes. Am I right?
>>
>> ctx.run(func) runs 'func' in the 'ctx' context.  Any changes to
>> ContextVars that func makes will stay isolated to the 'ctx' context.
>>
>> >
>> > Maybe add a short comment to explain that?
>>
>> Added.
>
>
> The PEP still contains the following paragraph:
>
>> Any changes to the context will be contained and persisted in the
>> ``Context`` object on which ``run()`` is called on.
>
> This phrase is confusing; it could be read as implying that context changes
> made by the function *will* get propagated back to the caller of run(),
> contradicting what was said earlier. Maybe it's best to just delete it?
> Otherwise if you intend it to add something it needs to be rephrased. Maybe
> "persisted" is the key word causing confusion?

I'll remove "persisted" now, I agree it adds more confusion than
clarity.  Victor is also confused with how 'Context.run()' is
currently explained, I'll try to make it clearer.

Thank you,
Yury