On Mon, Oct 16, 2017 at 11:49 AM, Guido van Rossum <gu...@python.org> wrote: > On Sun, Oct 15, 2017 at 10:26 PM, Nathaniel Smith <n...@pobox.com> wrote: >> >> On Sun, Oct 15, 2017 at 10:10 PM, Guido van Rossum <gu...@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.
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. The key arguments for keeping ContextVar abstraction: * 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. Yury _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
