[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"?)

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