[Python-ideas] PEP draft: context variables

Paul Moore p.f.moore at gmail.com
Sat Oct 14 15:47:42 EDT 2017

On 14 October 2017 at 17:50, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 14 October 2017 at 21:56, Paul Moore <p.f.moore at gmail.com> wrote:
> TL;DR of below: PEP 550 currently gives you what you're after, so your
> perspective counts as a preference for "please don't eagerly capture the
> creation time context in generators or coroutines".

Thank you. That satisfies my concerns pretty well.

> The suggestion has been made that we should instead be capturing the active
> context when "url_get(url)" is called, and implicitly switching back to that
> at the point where await is called. It doesn't seem like a good idea to me,
> as it breaks the "top to bottom" mental model of code execution (since the
> "await cr" expression would briefly switch the context back to the one that
> was in effect on the "cr = url_get(url)" line without even a nested suite to
> indicate that we may be adjusting the order of code execution).

OK. Then I think that's a bad idea - and anyone proposing it probably
needs to explain much more clearly why it might be a good idea to jump
around in the timeline like that.

> If you capture the context eagerly, then there are fewer opportunities to
> get materially different values from "data = list(iterable)" and "data =
> iter(context_capturing_iterable)".
> While that's a valid intent for folks to want to be able to express, I
> personally think it would be more clearly requested via an expression like
> "data = iter_in_context(iterable)" rather than having it be implicit in the
> way generators work (especially since having eager context capture be
> generator-only behaviour would create an odd discrepancy between generators
> and other iterators like those in itertools).

OK. I understand the point here - but I'm not sure I see the practical
use case for iter_in_context. When would something like that be used?


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