[Python-ideas] PEP draft: context variables

Steve Dower steve.dower at python.org
Fri Oct 13 12:48:55 EDT 2017

On 13Oct2017 0941, Yury Selivanov wrote:
> On Fri, Oct 13, 2017 at 3:25 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> [..]
>> However, considering that coroutines are almost always instantiated at the
>> point where they're awaited, I do concede that creation time context capture
>> would likely also work out OK for the coroutine case, which would leave
>> contextlib.contextmanager as the only special case (and it would turn off
>> both creation-time context capture *and* context isolation).
> Actually, capturing context at the moment of coroutine creation (in
> PEP 550 v1 semantics) will not work at all.  Async context managers
> will break.
>     class AC:
>         async def __aenter__(self):
>              pass
> ^ If the context is captured when coroutines are instantiated,
> __aenter__ won't be able to set context variables and thus affect the
> code it wraps.  That's why coroutines shouldn't capture context when
> created, nor they should isolate context.  It's a job of async Task.

Then make __aenter__/__aexit__ when called by "async with" an exception 
to the normal semantics?

It seems simpler to have one specially named and specially called 
function be special, rather than make the semantics more complicated for 
all functions.


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