Overlapping type

[srittau]

This is an rough idea from the recent discussion of whether to use object/Any or _KT for arguments of certain methods like pop or __sub__. E.g.:

class MyPoppable[KT, VT]:
    def pop(self, x: X) -> VT: ...

What should X be? Using KT (or KT | None) find type errors like the following:

poppable: MyPoppable[str]
poppable.pop(123)

But has a problem with overlapping types:

def foo(x: int | str):
    poppable.pop(x)

On the other hand, using object doesn't catch type errors like poppable.pop(123).

The solution would be able to specify that any partially overlapping type would be accepted:

class MyPoppable[KT, VT]:
    def pop(self, x: Overlapping[KT]) -> VT: ...

MyPoppable[str]().pop() would accept str, str | None, Literal["x"], str | int, etc., but not e.g. plain int.

[randolf-scholz]

I think it's debatable whether this is really a TypeError or rather just a statically knowable KeyError.
Overlapping[T] may be generally useful, though. I believe it can be emulated with intersections (#213) as follows:

class MyPoppable[KT, VT]:
    def pop[T](self, x: T & KT) -> VT: ...

poppable: MyPoppable[str]
poppable.pop(123)  # E: empty intersection

def foo(x: int | str):
    # OK, since (int | str) & str = (int & str) | (str & str) = Never | str = str
    # is non-empty.
    poppable.pop(x)