Implement the getitem method of a minimal collections.abc sequence with type hints

Question

How does a minimal implementation of a Sequence from collections.abc, together with the correct type hints, look like?

According to its documentation, __len__ and __getitem__ are sufficient. My type hinter complains about the implementation of __getitem__ though, although my implementation follows the python docs.

My current version is the following (with warnings from basedpyright):

from collections.abc import Sequence
from typing import override

class MySeq(Sequence[float]):
    def __init__(self):
        self._data: list[float] = list()

    @override
    def __len__(self) -> int:
        return len(self._data)

    @override
    def __getitem__(self, key) -> float: # Type annotation is missing for parameter "key"
        return self._data[key]           # Return type is unknown

How do I type hint key? Just key: int is not accepted by the type checker. It gives the warning: "slice[Any, Any, Any]" is not assignable to "int". What choices do I have here?
What is wrong with the return type? The full warning is "float" is not assignable to "Sequence[float]"

"Just key: int is not accepted by the type checker" - so what does it tell you? Mypy shows the missing overload; Sequences can also be sliced: docs.python.org/3/reference/datamodel.html#object.__getitem__ — jonrsharpe
– jonrsharpe, Commented Mar 18 at 16:44
@jonrsharpe See my edit. I added the override annotations as well — 502E532E
– 502E532E, Commented Mar 18 at 16:52
overload, not override - you can index into a Sequence with an int or a slice, therefore receiving either a float or a Sequence[float], as the errors tell you. — jonrsharpe
– jonrsharpe, Commented Mar 18 at 16:56
The problem is that you inherit the requirement to accept slice arguments from Sequence.__get_item__, even though nothing in the documentation indicates that you need to accept anything other than an integer key. (I suspect that is a shortcoming in the documentation, though, rather than an overreach by the signature defined in typeshed/stdlib/typing.pyi — chepner
– chepner, Commented Mar 18 at 17:12

Anerdw · Accepted Answer · 2025-04-29 14:31:43Z

3

It seems that although the __getitem__-docs list slice support as optional, the typing information requires both support for integer indices and slices. This contradiction seems to be known/not intended to be fixed, see this issue at the official python repository.

Implementing both resolves both warnings:

from collections.abc import Sequence
from typing import override, overload

class MySeq(Sequence[float]):
    def __init__(self):
        self._data: list[float] = list()

    @override
    def __len__(self) -> int:
        return len(self._data)

    @overload
    def __getitem__(self, key: int) -> float:
        pass

    @overload
    def __getitem__(self, key: slice) -> Sequence[float]:
        pass

    @override
    def __getitem__(self, key: int | slice) -> float | Sequence[float]:
        return self._data[key]

Thanks for the comments of @jonrsharpe and @chepner for providing the right direction with this.

edited Apr 29 at 14:31

Anerdw

2,8343 gold badges18 silver badges44 bronze badges

answered Mar 18 at 17:27

502E532E

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