How to implement overload of __setitem__ for MutableSequence?

[asottile]

I'm trying to implement my own MutableSequence but struggling with the proper way to implement the @overload

Here's my code example:

from typing import Iterable
from typing import List
from typing import MutableSequence
from typing import Union
from typing import overload


class RecordBehaviourList(MutableSequence[str]):
    def __init__(self, lst: List[str]) -> None:
        self._lst = lst

    @overload
    def __getitem__(self, index: int) -> str:
        ...

    @overload  # noqa: F811  # TODO: flake8 3.8
    def __getitem__(self, index: slice) -> MutableSequence[str]:
        ...

    def __getitem__(  # noqa: F811  # TODO: flake8 3.8
            self,
            index: Union[int, slice],
    ) -> Union[str, MutableSequence[str]]:
        return self._lst[index]

    @overload
    def __setitem__(self, index: int, value: str) -> None:
        ...

    @overload  # noqa: F811  # TODO: flake8 3.8
    def __setitem__(self, index: slice, value: Iterable[str]) -> None:
        ...

    def __setitem__(  # noqa: F811  # TODO: flake8 3.8
            self,
            index: Union[int, slice],
            value: Union[str, Iterable[str]],
    ) -> None:
        self._lst[index] = value

    def __delitem__(self, index: Union[int, slice]) -> None:
        del self._lst[index]

    def insert(self, index: int, value: str) -> None:
        self._lst.insert(index, value)

I've taken the overload definitions from this example (?) but perhaps I'm doing this incorrectly?

For now I can # type: ignore but seems non-ideal

Here's my version information and current output

$ mypy --version
mypy 0.740
$ mypy t3.py
t3.py:39: error: Invalid index type "Union[int, slice]" for "List[str]"; expected type "int"
t3.py:39: error: Incompatible types in assignment (expression has type "Union[str, Iterable[str]]", target has type "str")
Found 2 errors in 1 file (checked 1 source file)

[ilevkivskyi]

Mypy doesn't use overload information to type-check the body. For such cases you can use a trick:

if isinstance(index, int):
    assert isinstance(value, str)
    self._lst[index] = value
else:
    assert not isinstance(value, str)
    self._lst[index] = value

This is kind of ugly however. I think we can use the overload information and type-check body multiple times in simple cases, i.e. if neither *args nor **kwargs appear in any of the overloads and in the implementation.

cc @Michael0x2a

[Greedquest]

@ilevkivskyi Is there a way to do this within mypy which is a runtime no-op or as close as possible, perhaps with TypeGuard or cast or something?

[ilevkivskyi]

You can use if TYPE_CHECKING: ... for something to have effect in mypy but not at runtime.