[Python-Dev] PEP 550 v4

[Nathaniel Smith]

On Sat, Aug 26, 2017 at 10:25 AM, Eric Snow <ericsnowcurrently at gmail.com> wrote:
> With threads we have a directed graph of execution, rooted at the root
> thread, branching with each new thread and merging with each .join().
> Each thread gets its own copy of each threading.local, regardless of
> the relationship between branches (threads) in the execution graph.
>
> With async (and generators) we also have a directed graph of
> execution, rooted in the calling thread, branching with each new async
> call.  Currently there is no equivalent to threading.local for the
> async execution graph.  This proposal involves adding such an
> equivalent.
>
> However, the proposed solution isn''t quite equivalent, right?  It
> adds a concept of lookup on the chain of namespaces, traversing up the
> execution graph back to the root.  threading.local does not do this.
> Furthermore, you can have more than one threading.local per thread.
> From what I read in the PEP, each node in the execution graph has (at
> most) one Execution Context.
>
> The PEP doesn't really say much about these differences from
> threadlocals, including a rationale.  FWIW, I think such a COW
> mechanism could be useful.  However, it does add complexity to the
> feature.  So a clear explanation in the PEP of why it's worth it would
> be valuable.

You might be interested in these notes I wrote to motivate why we need
a chain of namespaces, and why simple "async task locals" aren't
sufficient:

    https://github.com/njsmith/pep-550-notes/blob/master/dynamic-scope.ipynb

They might be a bit verbose to include directly in the PEP, but
Yury/Elvis, feel free to steal whatever if you think it'd be useful.

-n

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