[Python-ideas] PEP draft: context variables

Paul Moore p.f.moore at gmail.com
Sun Oct 15 06:31:29 EDT 2017

On 15 October 2017 at 05:39, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 15 October 2017 at 05:47, Paul Moore <p.f.moore at gmail.com> wrote:
>> On 14 October 2017 at 17:50, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> > 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?
> Suppose you have some existing code that looks like this:
>     results = [calculate_result(a, b) for a, b in data]
> If calculate_result is context dependent in some way (e.g. a & b might be
> decimal values), then eager evaluation of "calculate_result(a, b)" will use
> the context that's in effect on this line for every result.
> Now, suppose you want to change the code to use lazy evaluation, so that you
> don't need to bother calculating any results you don't actually use:
>     results = (calculate_result(a, b) for a, b in data)
> In a PEP 550 world, this refactoring now has a side-effect that goes beyond
> simply delaying the calculation: since "calculate_result(a, b)" is no longer
> executed immediately, it will default to using whatever execution context is
> in effect when it actually does get executed, *not* the one that's in effect
> on this line.
> A context capturing helper for iterators would let you decide whether or not
> that's what you actually wanted by instead writing:
>     results = iter_in_context(calculate_result(a, b) for a, b in data)
> Here, "iter_in_context" would indicate explicitly to the reader that
> whenever another item is taken from this iterator, the execution context is
> going to be temporarily reset back to the way it was on this line. And since
> it would be a protocol based iterator-in-iterator-out function, you could
> wrap it around *any* iterator, not just generator-iterator objects.

OK, got it. That sounds to me like a candidate for a stdlib function
(either because it's seen as a common requirement, or because it's
tricky to get right - or both). The PEP doesn't include it, as far as
I can see, though.

But I do agree with MAL, it seems wrong to need a helper for this,
even though it's a logical consequence of the other semantics I
described as intuitive :-(


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