[Python-Dev] Timeout for PEP 550 / Execution Context discussion

Guido van Rossum guido at python.org
Mon Oct 16 13:35:17 EDT 2017

On Mon, Oct 16, 2017 at 9:53 AM, Yury Selivanov <yselivanov.ml at gmail.com>

> I think we can still implement context isolation in generators in
> later versions for ContextVars.  In 3.7, ContextVars will only support
> async tasks and threads.  Using them in generators will be
> *documented* as unsafe, as the context will "leak out".  Fixing
> generators in some later version of Python will then be a feature/bug
> fix.  I expect almost no backwards compatibility issue, same as I
> wouldn't expect them if we switched decimal to PEP 550 in 3.7.

Context also leaks into a generator. That's a feature too. Basically a
generator does not have its own context; in that respect it's no different
from a regular function call. The apparent difference is that it's possible
to call next() on a generator object from different contexts (that's always
been possible, in today's Python you can do this from multiple threads and
there's even protection against re-entering a generator frame that's
already active in another thread -- the GIL helps here of course).

I expect that any future (post-3.7) changes to how context work in
generators will have to support this as the default behavior, and to get
other behavior the generator will have to be marked or wrapped somehow.

--Guido van Rossum (python.org/~guido)
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