[Python-Dev] PEP 567 -- Context Variables

[Yury Selivanov]

Hi Ben,

On Sun, Dec 17, 2017 at 10:38 AM, Ben Darnell <ben at bendarnell.com> wrote:
> On Tue, Dec 12, 2017 at 12:34 PM Yury Selivanov <yselivanov.ml at gmail.com>
> wrote:
>>
>> Hi,
>>
>> This is a new proposal to implement context storage in Python.
>>
>> It's a successor of PEP 550 and builds on some of its API ideas and
>> datastructures.  Contrary to PEP 550 though, this proposal only focuses
>> on adding new APIs and implementing support for it in asyncio.  There
>> are no changes to the interpreter or to the behaviour of generator or
>> coroutine objects.
>
>
> I like this proposal. Tornado has a more general implementation of a similar
> idea
> (https://github.com/tornadoweb/tornado/blob/branch4.5/tornado/stack_context.py),
> but it also tried to solve the problem of exception handling of
> callback-based code so it had a significant performance cost (to interpose
> try/except blocks all over the place). Limiting the interface to
> coroutine-local variables should keep the performance impact minimal.

Thank you, Ben!

Yes, task local API of PEP 567 has no impact on generators/coroutines.
Impact on asyncio should be well within 1-2% slowdown, visible only in
microbenchmarks (and asyncio will be 3-6% faster in 3.7 at least due
to some asyncio.Task C optimizations).

[..]
> One caveat based on Tornado's experience with stack_context: There are times
> when the automatic propagation of contexts won't do the right thing (for
> example, a database client with a connection pool may end up hanging on to
> the context from the request that created the connection instead of picking
> up a new context for each query).

I can see two scenarios that could lead to that:

1. The connection pool explicitly captures the context with 'get_context()' at
the point where it was created. It later schedules all of its code within the
captured context with Context.run().

2. The connection pool calls ContextVar.get() once and _caches_ it.

Both (1) and (2) are anti-patterns.  The documentation of asyncio and
contextvars
module will explain that users are supposed to simply call
ContextVar.get() whenever
they need to get a context value (e.g. there's no need to
cache/persist it) and that
 they should not manage the context manually (just trust asyncio to do
that for you).

Thank you,
Yury