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

Guido van Rossum guido at python.org
Mon Oct 16 13:00:14 EDT 2017

On Mon, Oct 16, 2017 at 9:11 AM, Yury Selivanov <yselivanov.ml at gmail.com>

> On Mon, Oct 16, 2017 at 11:49 AM, Guido van Rossum <guido at python.org>
> wrote:
> > On Sun, Oct 15, 2017 at 10:26 PM, Nathaniel Smith <njs at pobox.com> wrote:
> >> 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.
> Yeah, we can do a proxy.
> >> - 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.
> Correct.
> >> 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"?)
> Question: why do we want EC objects to be mappings?  I'd rather make
> them opaque, which will result in less code and make it more
> future-proof.

I'd rather have them mappings, since that's what they represent. It helps
users understand what's going on behind the scenes, just like modules,
classes and (most) instances have a `__dict__` that you can look at and (in
most cases) manipulate.

> The key arguments for keeping ContextVar abstraction:

To be clear, I do want to keep ContextVar!

> * Naturally avoids name clashes.
> * Allows to implement efficient caching.  This is important if we want
> libraries like decimal/numpy to start using it.
> * Abstracts away the actual implementation of the EC.  This is a
> future-proof solution, with which we can enable EC support for
> generators in the future.  We already know two possible solutions (PEP
> 550 v1, PEP 550 current), and ContextVar is a good enough abstraction
> to support both of them.
> IMO ContextVar.set() and ContextVar.get() is a simple and nice API to
> work with the EC.  Most people (aside framework authors) won't even
> need to work with EC objects directly anyways.

Sure. But (unlike you, it seems) I find it important that users can
understand their actions in terms of operations on the mapping representing
the context. Its type should be a specific class that inherits from
`MutableMapping[ContextVar, object]`.

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