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

Nathaniel Smith njs at pobox.com
Mon Oct 16 20:29:19 EDT 2017

On Mon, Oct 16, 2017 at 11:12 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
> What would be really nice is to have attribute access like thread locals.
> Instead of working with individual ContextVars you grab the LocalContext and
> access the vars as attributes.  I don't recall reading in the PEP why this
> is a bad idea.

You're mixing up levels -- the way threading.local objects work is
that there's one big dict that's hidden inside the interpreter (in the
ThreadState), and it holds a separate little dict for each
threading.local. The dict holding ContextVars is similar to the big
dict; a threading.local itself is like a ContextVar that holds a dict.
(And the reason it's this way is that it's easy to build either
version on top of the other, and we did some survey of threading.local
usage and the ContextVar style usage was simpler in the majority of

For threading.local there's no way to get at the big dict at all from
Python; it's hidden inside the C APIs and threading internals. I'm
guessing you've never missed this :-). For ContextVars we can't hide
it that much, because async frameworks need to be able to swap the
current dict when switching tasks and clone it when starting a new
task, but those are the only absolutely necessary operations.


Nathaniel J. Smith -- https://vorpus.org

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