On Thu, Jan 4, 2018 at 9:42 PM, Guido van Rossum <gu...@python.org> wrote: > On Thu, Jan 4, 2018 at 7:58 PM, Nathaniel Smith <n...@pobox.com> wrote: >> This does make me think that I should write up a short PEP for >> extending PEP 567 to add context lookup, PEP 550 style: it can start >> out in Status: deferred and then we can debate it properly before 3.8, >> but at least having the roadmap written down now would make it easier >> to catch these details. (And it might also help address Paul's >> reasonable complaint about "unstated requirements".) > > Anything that will help us kill a 550-pound gorilla sounds good to me. :-) > > It might indeed be pretty short if we follow the lead of ChainMap (even > using a different API than MutableMapping to mutate it). Maybe > copy_context() would map to new_child()? Using ChainMap as a model we might > even avoid the confusion between Lo[gi]calContext and ExecutionContext which > was the nail in PEP 550's coffin. The LC associated with a generator in PEP > 550 would be akin to a loose dict which can be pushed on top of a ChainMap > using cm = cm.new_child(<dict>). (Always taking for granted that instead of > an actual dict we'd use some specialized mutable object implementing the > Mapping protocol and a custom mutation protocol so it can maintain > ContextVar cache consistency.)
The approach I took in PEP 568 is even simpler, I think. The PEP is a few pages long because I wanted to be exhaustive to make sure we weren't missing any details, but the tl;dr is: The ChainMap lives entirely inside the threadstate, so there's no need to create a LC/EC distinction -- users just see Contexts, or there's the one stack introspection API, get_context_stack(), which returns a List[Context]. Instead of messing with new_child, copy_context is just Context(dict(chain_map)) -- i.e., it creates a flattened copy of the current mapping. (If we used new_child, then we'd have to have a way to return a ChainMap, reintroducing the LC/EC mess. Plus it would allow for changes to the Context's inside one ChainMap to "leak" into the child or vice-versa.) And Context push/pop is just push/pop on the threadstate's ChainMap's '.maps' attribute. -n -- Nathaniel J. Smith -- https://vorpus.org _______________________________________________ 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
