[Python-ideas] (PEP 555 subtopic) Propagation of context in async code

Koos Zevenhoven k7hoven at gmail.com
Sun Oct 15 11:11:56 EDT 2017

On Sun, Oct 15, 2017 at 5:34 PM, Amit Green <amit.mixie at gmail.com> wrote:

> On Sun, Oct 15, 2017 at 9:44 AM, Koos Zevenhoven <k7hoven at gmail.com>
> wrote:
>> So, see below for some more discussion between (it would be useful if
>> some people could reply to this email and say if and why they agree or
>> disagree with something below -- also non-experts that roughly understand
>> what I'm talking about):
> Yes, I understand what you are roughly talking about.
> Also, yes, generators are co-routines [though when starting to work with
> generators, people don't fully realize this].
> But then how to address "my" original problem where the context would
>> propagate through awaits, and next/send? From what others have written, it
>> seems there are also other situations where that is desired. There are
>> several ways to solve the problem as an extension to PEP 555, but below is
>> one:
>>> ​We need both versions: the one that propagates first_context() into the
>>> coroutine, and the one that propagates second_context() into it. Or, using
>>> my metaphor from the other thread, we need "both the forest and the trees".
>>> ​A solution to this would be to have two types of context arguments:
>>> 1. (calling) context arguments​
>>> and
>>> 2. execution context arguments
>> So yes, I'm actually serious about this possibility. Now it would be up
>> to library and framework authors to pick the right variant of the two. And
>> this is definitely something that could be documented very clearly.​
> This is an interesting idea.  I would add you also need:
> 3.  Shared context, the generator shares the context with it's caller
> which means:
>    - If the caller changes the context, the generator, see the changed
>    context next time it's __next__ function is called
>    - If the generator changes the context, the caller sees the changed
>    context.
>    - [This clearly make changing the context using 'with' totally
>    unusable in both the caller & the generator -- unless we add even odder
>    semantics, that the generator restores the original context when it
>    exists???]
>    - (As per previous email by me, I claim this is the most natural way
>    beginners are going to think it works; and needs to be supported; also in
>    real code this is not often useful]
>    - I'm not sure if this would even work with async or not -- *IF* not,
>    I would still have a syntax for the user to attempt this -- and throw a
>    Syntax Error when they do, with a good explanation of why this combination
>    doesn't work for async.  I believe good explanations are a great way for
>    people to learn which features can't be combined together & why.
Just as a quick note, after skimming through your bullet points: ​All of
this is indeed covered with decorators and ​other explicit mechanisms in
the PEP 555 approach. I don't think we need syntax errors, though.

>> If something was not clear, but seems relevant to what I'm trying to
>> discuss here, please ask :)
> I looked for you quote "we need both the forest & the trees", but didn't
> find it here.  I quite strongly agree we need both; in fact need also the
> third case I highlighted above.
The ordering of the archive was indeed thoroughly destroyed. Ordering by
date might help. ​But the quote you ask for is here:



> As for what Guido wrote, that we might be trying to solve too many
> problems -- probably.  However, these are real issues with context's, not
> edge cases.
> Thus Guido writing we don't want to allow yield within a 'with' clause (as
> it leaks context) .. I would argue two things:
>    - There are use cases where we *DO* want this -- rare -- true -- but
>    they exist (i.e.: my #3 above)
>    - IF, for simplicity, sake, it is decided not to handle this case now;
>    then make it a syntax error in the language; i.e.:
>       def f():
>          with context() as x:
>             yield 1
>    Syntax error: 'yield' may not be used inside a 'with' clause.
> This would really help new users not to make a mistake that takes hours to
> debug; & help correct their [initial mistaken] thinking on how contexts &
> generators interact.

+ Koos Zevenhoven + http://twitter.com/k7hoven +
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