fix list concatenation

[JelleZijlstra]

Fixes #2383, python/mypy#5492.

I was a little surprised this works, but it does. I tried the test cases from both of the linked issues, and mypy allows them without any errors when this typeshed patch is applied.

Unfortunately, with this change __iadd__ becomes incompatible with __add__ according to mypy. I tried changing it to def __iadd__(self: List[_T_co], x: Iterable[_T_co]) -> List[_T_co]: ..., which passes mypy, but that breaks this test case:

from typing import List
x: List[object]
y: List[int]
x += y

Therefore, I chose to # type: ignore it instead.

[ilevkivskyi]

I am not sure if this is the right solution. Maybe we can try something closer to set.__or__/set.__ior__? Will this fix the issues?

I guess a perfect solution for this kind of questions would require python/mypy#2354 solved first.

[JukkaL]

The return type needs to use a union, similar to the set operations.

I don't think that generic self types would help, since subclasses will default to generating list instances:

class A(list): pass
type(A([1]) + A([1])) is list  # True

[ilevkivskyi]

I don't think that generic self types would help, since subclasses will default to generating list instances

Yes, indeed, it returns just a list. I remember people wanted generic self types when they want the return to be a subtype, but it is not the case here.

The return type needs to use a union, similar to the set operations.

This actually might not solve the original issue, where a join is preferred:

class B: ...
class C1(B): ...
class C2(B): ...

lst: List[B] = [C1()] + [C2()]

This is however an eternal question of union vs join, so I think there is no ideal solution here.

[ilevkivskyi]

Just to clarify I do think returning a List[Union[T, S]] is a better compromise.

[JelleZijlstra]

I am going to be out for another week and won't have time to update the code in this PR. If it's urgent, feel free to take over the PR and change the code as you see fit; otherwise I'll implement your suggestion in a few weeks.

[JelleZijlstra]

I implemented the Union return type suggestion now.

[ilevkivskyi]

LGTM!