On 13 October 2017 at 19:32, Yury Selivanov email@example.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).
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
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:
 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 ;-))