Minimal reproducer:

>>> class A:
...     def __round__(self):
...         return NotImplemented
...
>>> round(A())
NotImplemented

Should give a TypeError.

This can be useful when deciding, for example, if a given a.dtype implements round based on the dtype

NotImplemented is documented as only being used for binary operators: https://docs.python.org/3/library/constants.html#NotImplemented

Changing that seems like a pretty large issue. I'd suggest discussing this on python-ideas.

Maybe better for your situation is to actually raise NotImplementedError yourself, or ValaueError if that's what you really want to see.

Changing version numbers, since this feature could only go in to 3.9.

If you want to get a TypeError, raise a TypeError.

Agreed with Eric and Serhiy. This seems to be a misunderstanding about the scope of NotImplemented.

@Hameer Abbasi: if you have an example of a problem that would be solved by this behaviour change, and which can't be solved simply by raising TypeError or another exception within __round__, feel free to open a discussion on the python-ideas mailing list.