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
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?"float" is not assignable to "Sequence[float]"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.