Brandt Bucher <brandtbuc...@gmail.com> added the comment:

Any class that registers itself as a Mapping/MutableMapping using their 
"register" methods (*not* through inheritance) will be lacking the new methods. 
This includes all C extensions.

Further, binary "|" would require subclasses of these ABCs to have a concrete 
way of creating a new instance (we don't have anything like Set._from_iterable 
for this). One option could be to just call self.__class__({**self, **other}), 
but a cursory scan of the stdlib shows that many classes don't have compatible 
signatures, and the we would get lots of surprising/broken behavior if we just 
suddenly introduced this.

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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue43085>
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