[Python-Dev] Timeout for PEP 550 (original) (raw)

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

Guido van Rossum guido at python.org
Tue Oct 17 16:16:19 EDT 2017

On Tue, Oct 17, 2017 at 12:51 PM, Nathaniel Smith <njs at pobox.com> wrote:

On Oct 17, 2017 11:25 AM, "Guido van Rossum" <guido at python.org> wrote:

In short, I really don't think there's a need for context variables to be faster than instance variables. There really is: currently the cost of looking up a thread local through the C API is a dict lookup, which is faster than instance variable lookup, and decimal and numpy have both found that that's already too expensive.

(At first I found this hard to believe, but then I realized that decimal and numpy presumably access these from C code where a typical operation like Decimal.add is much faster than a dict lookup. So point taken.)

Or maybe you're just talking about the speed when the cache misses, in which case never mind :-).

I'm happy to support caching the snot out of this, but it seems we agree that the "semantics" can be specified without taking the caching into account, and that's what I'm after. I presume that each ContextVar object will have one cached value? Because that's what PEP 550 currently specifies. Surely it wouldn't be hard for a direct setitem (or delitem) call to cause the cache to be invalidated, regardless of whether the affected context is a current context or not (a bunch of different approaches suggest themselves).Oother mutating methods can be implemented in terms of setitem (or delitem), and I don't care for them to be as fast as a cached lookup.

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