mypy: Signature of "__getitem__" incompatible with supertype "Sequence"

Question

I have a class that inherits from MutableSequence like this:

class QqTag(MutableSequence):
    def __init__(self):
        self._children = []
    def __getitem__(self, idx: int) -> 'QqTag':
        return self._children[idx]

mypy complains that Signature of "__getitem__" incompatible with supertype "Sequence".

In Sequence, this method is defined as:

@abstractmethod
def __getitem__(self, index):
    raise IndexError

So, what's the problem and why mypy isn't happy with my implementation?

Answer 1

Another option is to use the typing.overload decorator.

from typing import overload

class QqTag(MutableSequence):
    def __init__(self):
        self._children = []
    @overload
    def __getitem__(self, i: int) -> 'QqTag':
        ...
    @overload
    def __getitem__(self, s: slice) -> list['QqTag']:
        ...
    def __getitem__(self, idx):
        return self._children[idx]

See examples from typing or PEP 484 -- Type Hints.

Answer 2

As mentioned in comments, a typeof slice can also be passed. Ie, change idx: int to idx: Union[int, slice].

This will make mypy happy (at least on my machine ;):

class QqTag(MutableSequence):
    def __init__(self):
        self._children = []

    def __getitem__(self, idx: Union[int, slice]) -> 'QqTag':
        return self._children[idx]