Use abstract types from numbers

[sobolevn]

Right now types declared in numbers.pyi are not used.
This causes this problem:

from numbers import Real

def test(f: Real) -> Real:
    return f

test(1.32)
# error: Argument 1 to "test" has incompatible type "float"; expected "Real"

But, in runtime:

• float is registered to be Real, source: https://github.com/python/cpython/blob/12360aa159c42c7798fd14225d271e6fd84db7eb/Lib/numbers.py#L264
  Also:
• complex: https://github.com/python/cpython/blob/12360aa159c42c7798fd14225d271e6fd84db7eb/Lib/numbers.py#L144
• int: https://github.com/python/cpython/blob/12360aa159c42c7798fd14225d271e6fd84db7eb/Lib/numbers.py#L393

But, how can we type it? Options:

1. Use a base class in builtins.pyi
2. Use Real.register(float) in builtins.pyi
3. Use Real.register(float) in numbers.pyi (as runtime does)

[srittau]

I don't think we can really do anything in typeshed. This is best handled on the type checker level.

[hauntsaninja]

An approach like python/mypy#3186 (comment) could be feasible.

[srittau]

A typing feature that allows stubs to register a type with another type, similar to what abcs provide at runtime, would be useful for those legacy features, though.

[AlexWaygood]

I invested some energy a while back into seeing if you could do this, and I think the honest answer is that there's no way to make it work at typeshed. Mypy now gives a much more helpful error message if you try to use an abstract type from numbers these days, following python/mypy#15137.

Closing as per the rationale in python/mypy#3186 (comment)