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

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

Guido van Rossum guido at python.org
Mon Oct 16 11:49:43 EDT 2017

On Sun, Oct 15, 2017 at 10:26 PM, Nathaniel Smith <njs at pobox.com> wrote:

On Sun, Oct 15, 2017 at 10:10 PM, Guido van Rossum <guido at python.org> wrote: > Yes, that's what I meant by "ignoring generators". And I'd like there to be > a "current context" that's a per-thread MutableMapping with ContextVar keys. > Maybe there's not much more to it apart from naming the APIs for getting and > setting it? To be clear, I am fine with this being a specific subtype of > MutableMapping. But I don't see much benefit in making it more abstract than > that.

We don't need it to be abstract (it's fine to have a single concrete mapping type that we always use internally), but I think we do want it to be opaque (instead of exposing the MutableMapping interface, the only way to get/set specific values should be through the ContextVar interface). The advantages are: - This allows C level caching of values in ContextVar objects (in particular, funneling mutations through a limited API makes cache invalidation much easier)

Well the MutableMapping could still be a proxy or something that invalidates the cache when mutated. That's why I said it should be a single concrete mapping type. (It also doesn't have to derive from MutableMapping -- it's sufficient for it to be a duck type for one, or perhaps some Python-level code could register() it.

- It gives us flexibility to change the underlying data structure without breaking API, or for different implementations to make different choices -- in particular, it's not clear whether a dict or HAMT is better, and it's not clear whether a regular dict or WeakKeyDict is better.

I would keep it simple and supid, but WeakKeyDict is a subtype of MutableMapping, and I'm sure we can find a way to implement the full MutableMapping interface on top of HAMT as well.

The first point (caching) I think is the really compelling one: in practice decimal and numpy are already using tricky caching code to reduce the overhead of accessing the ThreadState dict, and this gets even trickier with context-local state which has more cache invalidation points, so if we don't do this in the interpreter then it could actually become a blocker for adoption. OTOH it's easy for the interpreter itself to do this caching, and it makes everyone faster.

I agree, but I don't see how making the type a subtype (or duck type) of MutableMapping prevents any of those strategies. (Maybe you were equating MutableMapping with "subtype of dict"?)

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