Yury Selivanov <yseliva...@gmail.com> added the comment:

> Would there be too much overhead if allowing specification of a python 
> function that contextvars calls on context changes?

Potentially yes, especially if we allow more than one context change callback.  
Allowing just one makes the API inflexible (what if you want to use two 
libraries from PyPI that both want to use the callback).  Allowing multiple 
context change callbacks leads to complicated API.

For extra context: context switches occur on every callback invocation in 
asyncio and there can be thousands of them per seconds (or even more). Adding 
any extra code to context switching code will noticeably degrade the 
performance.

In general, I'd suggest patching the C library to make state management 
customizable (like CPython allows you to customize which memory allocator to 
use)

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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue39660>
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