[UserList]: Return type for slice is set to whatever self is

[raabf]

When the input of __getitem__ is a slice, it returns an instance of the class whatever self currently is:

def __getitem__(self, i):
    if isinstance(i, slice):
        return self.__class__(self.data[i])

https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/Lib/collections/__init__.py#L1215-L1217

That would be at least an instance of UserList (and not MutableSequence like currently stated in typeshed) but could be also a subclass. So I changed the return type to an instance of the class of self.

Side note: That problem is already addressed in the python2 typesheds:

typeshed/stdlib/@python2/UserList.pyi

Line 18 in f527e96

def __getitem__(self: _S, s: slice) -> _S: ...

[srittau]

We actually have a fairly new Self alias for that, which you can import from _typeshed:

from _typeshed import Self

[...]

@overload
    def __getitem__(self: Self, i: slice) -> Self: ...

[raabf]

Oh that would be a very comfortable way. But does that work? Self from _typeshed is defined as Self = TypeVar("Self") (implicitly bound=object) and not TypeVar("_UserListT", bound=UserList) and the UserList.__getitem__ implementation is accessing the attribute self.data, and the upper bound of self namely object does not have this attribute.

[srittau]

A typevar annotating self is implicitly bound to that class. I'm not sure this is documented anywhere, but this is how type checkers work.

[raabf]

At least I never read that, and hence did not know about that, … but that behaviour makes life easier. Thanks for explanation! I changed the PR to _typesheld.Self.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉