Hi Ben, On Sun, Dec 17, 2017 at 10:38 AM, Ben Darnell <b...@bendarnell.com> wrote: > On Tue, Dec 12, 2017 at 12:34 PM Yury Selivanov <yselivanov...@gmail.com> > wrote: >> >> Hi, >> >> This is a new proposal to implement context storage in Python. >> >> It's a successor of PEP 550 and builds on some of its API ideas and >> datastructures. Contrary to PEP 550 though, this proposal only focuses >> on adding new APIs and implementing support for it in asyncio. There >> are no changes to the interpreter or to the behaviour of generator or >> coroutine objects. > > > I like this proposal. Tornado has a more general implementation of a similar > idea > (https://github.com/tornadoweb/tornado/blob/branch4.5/tornado/stack_context.py), > but it also tried to solve the problem of exception handling of > callback-based code so it had a significant performance cost (to interpose > try/except blocks all over the place). Limiting the interface to > coroutine-local variables should keep the performance impact minimal.
Thank you, Ben! Yes, task local API of PEP 567 has no impact on generators/coroutines. Impact on asyncio should be well within 1-2% slowdown, visible only in microbenchmarks (and asyncio will be 3-6% faster in 3.7 at least due to some asyncio.Task C optimizations). [..] > One caveat based on Tornado's experience with stack_context: There are times > when the automatic propagation of contexts won't do the right thing (for > example, a database client with a connection pool may end up hanging on to > the context from the request that created the connection instead of picking > up a new context for each query). I can see two scenarios that could lead to that: 1. The connection pool explicitly captures the context with 'get_context()' at the point where it was created. It later schedules all of its code within the captured context with Context.run(). 2. The connection pool calls ContextVar.get() once and _caches_ it. Both (1) and (2) are anti-patterns. The documentation of asyncio and contextvars module will explain that users are supposed to simply call ContextVar.get() whenever they need to get a context value (e.g. there's no need to cache/persist it) and that they should not manage the context manually (just trust asyncio to do that for you). Thank you, 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
