[Python-ideas] PEP 550 v2

[Yury Selivanov]

There's another "major" problem with theading.local()-like API for PEP
550: C API.

threading.local() in C right now is PyThreadState_GetDict(), which
returns a dictionary for the current thread, that can be
queried/modified with PyDict_* functions.  For PEP 550 this would not
work.

The advantage of the current ContextKey solution is that the Python
API and C API are essentially the same: [1]

Another advantage, is that ContextKey implements a better caching,
because it can have only one value cached in it, see [2] for details.

[1] https://www.python.org/dev/peps/pep-0550/#new-apis
[2] https://www.python.org/dev/peps/pep-0550/#contextkey-get-cache

Yury