[Python-ideas] PEP draft: context variables

Guido van Rossum guido at python.org
Tue Oct 10 12:52:39 EDT 2017

On Tue, Oct 10, 2017 at 5:34 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 10 October 2017 at 01:24, Guido van Rossum <guido at python.org> wrote:
>> 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:
>>>> ​​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.
> The compatibility concern is that we want developers of existing libraries
> to be able to transparently switch from using thread local storage to
> context local storage, and the way thread locals interact with generators
> means that decimal (et al) currently use the thread local state at the time
> when next() is called, *not* when the generator is created.

Apart from the example in PEP 550, is that really a known idiom?

> I like Yury's example for this, which is that the following two examples
> are currently semantically equivalent, and we want to preserve that
> equivalence:
>     with decimal.localcontext() as ctx:
>         ctc.prex = 30
>         for i in gen():
>            pass
>     g = gen()
>     with decimal.localcontext() as ctx:
>         ctc.prex = 30
>         for i in g:
>           pass

Do we really want that equivalence? It goes against the equivalence from
Koos' example.

> The easiest way to maintain that equivalence is to say that even though
> preventing state changes leaking *out* of generators is considered a
> desirable change, we see preventing them leaking *in* as a gratuitous
> backwards compatibility break.

I dunno, I think them leaking in in the first place is a dubious feature,
and I'm not too excited that the design of the way forward should bend over
backwards to be compatible here.

The only real use case I've seen so far (not counting examples that just
show how it works) is Nathaniel's timeout example (see point 9 in Nathaniel’s
and I'm still not convinced that that example is important enough to
support either.

It would all be easier to decide if there were use cases that were less
far-fetched, or if the far-fetched use cases would be supportable with a
small tweak. As it is, it seems that we could live in a simpler, happier
world if we gave up on context values leaking in via next() etc. (I still
claim that in that case we wouldn't need chained lookup in the exposed
semantics, just fast copying of contexts.)

> This does mean that *neither* form is semantically equivalent to eager
> extraction of the generator values before the decimal context is changed,
> but that's the status quo, and we don't have a compelling justification for
> changing it.

I think the justification is that we could have a *significantly* simpler
semantics and implementation.

> If folks subsequently decide that they *do* want "capture on creation" or
> "capture on first iteration" semantics for their generators, those are easy
> enough to add as wrappers on top of the initial thread-local-compatible
> base by using the same building blocks as are being added to help event
> loops manage context snapshots for coroutine execution.

(BTW Capture on first iteration sounds just awful.)

I think we really need to do more soul-searching before we decide that a
much more complex semantics and implementation is worth it to maintain
backwards compatibility for leaking in via next().

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