[Python-ideas] PEP draft: context variables

[Guido van Rossum]

On Sun, Oct 8, 2017 at 11:46 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 8 October 2017 at 08:40, Koos Zevenhoven <k7hoven at gmail.com> wrote:
>
>> On Sun, Oct 8, 2017 at 12:16 AM, Nathaniel Smith <njs at pobox.com> wrote:
>>
>>> On Oct 7, 2017 12:20, "Koos Zevenhoven" <k7hoven at gmail.com> wrote:
>>>
>>>
>>> ​Unfortunately, we actually need a third kind of generator semantics,
>>> something like this:
>>>
>>> @​contextvars.caller_context
>>> def genfunc():
>>>     assert cvar.value is the_value
>>>     yield
>>>     assert cvar.value is the_value
>>>
>>> with cvar.assign(the_value):
>>>     gen = genfunc()
>>>
>>> next(gen)
>>>
>>> with cvar.assign(1234567890):
>>>     try:
>>>         next(gen)
>>>     except StopIteration:
>>>         pass
>>>
>>> Nick, Yury and I (and Nathaniel, Guido, Jim, ...?) somehow just narrowly
>>> missed the reasons for this in discussions related to PEP 550. Perhaps
>>> because we had mostly been looking at it from an async angle.
>>>
>>>
>>> That's certainly a semantics that one can write down (and it's what the
>>> very first version of PEP 550 did),
>>>
>>
>> ​​I do remember Yury mentioning that the first draft of PEP 550 captured
>> something when the generator function was called. I think I started reading
>> the discussions after that had already been removed, so I don't know
>> exactly what it was. But I doubt that it was *exactly* the above, because
>> PEP 550 uses set and get operations instead of "assignment contexts" like
>> PEP 555 (this one) does. ​​
>>
>
> We didn't forget it, we just don't think it's very useful.
>

I'm not sure I agree on the usefulness. Certainly a lot of the complexity
of PEP 550 exists just to cater to Nathaniel's desire to influence what a
generator sees via the context of the send()/next() call. I'm still not
sure that's worth it. In 550 v1 there's no need for chained lookups.

-- 
--Guido van Rossum (python.org/~guido)
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