[Python-ideas] PEP draft: context variables

Paul Moore p.f.moore at gmail.com
Fri Oct 13 16:29:22 EDT 2017

On 13 October 2017 at 19:32, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
>>> It seems simpler to have one specially named and specially called function
>>> be special, rather than make the semantics
>>> more complicated for all functions.
> It's not possible to special case __aenter__ and __aexit__ reliably
> (supporting wrappers, decorators, and possible side effects).
>> +1.  I think that would make it much more usable by those of us who are not
>> experts.
> I still don't understand what Steve means by "more usable", to be honest.

I'd consider myself a "non-expert" in async. Essentially, I ignore it
- I don't write the sort of applications that would benefit
significantly from it, and I don't see any way to just do "a little
bit" of async, so I never use it.

But I *do* see value in the context variable proposals here - if only
in terms of them being a way to write my code to respond to external
settings in an async-friendly way. I don't follow the underlying
justification (which is based in "we need this to let things work with
async/coroutines) at all, but I'm completely OK with the basic idea
(if I want to have a setting that behaves "naturally", like I'd expect
decimal contexts to do, it needs a certain amount of language support,
so the proposal is to add that). I'd expect to be able to write
context variables that my code could respond to using a relatively
simple pattern, and have things "just work". Much like I can write a
context manager using @contextmanager and yield, and not need to
understand all the intricacies of __enter__ and __exit__. (BTW,
apologies if I'm mangling the terminology here - write it off as part
of me being "not an expert" :-))

What I'm getting from this discussion is that even if I *do* have a
simple way of writing context variables, they'll still behave in ways
that seem mildly weird to me (as a non-async user). Specifically, my
head hurts when I try to understand what that decimal context example
"should do". My instincts say that the current behaviour is wrong -
but I'm not sure I can explain why. So on that example, I'd ask the
following of any proposal:

1. Users trying to write a context variable[1] shouldn't have to jump
through hoops to get "natural" behaviour. That means that suggestions
that the complexity be pushed onto decimal.context aren't OK unless
it's also accepted that the current behaviour is wrong, and the only
reason decimal.context needs to replicated is for backward
compatibility (and new code can ignore the problem).
2. The proposal should clearly establish what it views as "natural"
behaviour, and why. I'm not happy with "it's how decimal.context has
always behaved" as an explanation. Sure, people asking to break
backward compatibility should have a good justification, but equally,
people arguing to *preserve* an unintuitive current behaviour in new
code should be prepared to explain why it's not a bug. To put it
another way, context variables aren't required to be bug-compatible
with thread local storage.

[1] I'm assuming here that "settings that affect how a library behave"
is a common requirement, and the PEP is intended as the "one obvious
way" to implement them.

Nick's other async refactoring example is different. If the two forms
he showed don't behave identically in all contexts, then I'd consider
that to be a major problem. Saying that "coroutines are special" just
reads to me as "coroutines/async are sufficiently weird that I can't
expect my normal patterns of reasoning to work with them". (Apologies
if I'm conflating coroutines and async incorrectly - as a non-expert,
they are essentially indistinguishable to me). I sincerely hope that
isn't the message I should be getting - async is already more
inaccessible than I'd like for the average user.

The fact that Nick's async example immediately devolved into a
discussion that I can't follow at all is fine - to an extent. I don't
mind the experts debating implementation details that I don't need to
know about. But if you make writing context variables harder, just to
fix Nick's example, or if you make *using* async code like (either of)
Nick's forms harder, then I do object, because that's affecting the
end user experience.

In that context, I take Steve's comment as meaning "fiddling about
with how __aenter__ and __aexit__ work is fine, as that's internals
that non-experts like me don't care about - but making context
variables behave oddly because of this is *not* fine".

Apologies if the above is unhelpful. I've been lurking but not
commenting here, precisely because I *am* a non-expert, and I trust
the experts to build something that works. But when non-experts were
explicitly mentioned, I thought my input might be useful.

The following quote from the Zen seems particularly relevant here:

    If the implementation is hard to explain, it's a bad idea.

(although the one about needing to be Dutch to understand why
something is obvious might well trump it ;-))


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