[Python] How to type-annotate write when subclassing io.RawIOBase, getting "Liskov...

I want to type-annotate the write in a class that subclasses io.RawIOBase.

I'm struggling to get anything other than Any to type check, which is frustrating, because I should be able to use a much more specific type here.

The following doesn't type check unfortunately:

from io import RawIOBase
from typing import Union

class BytesWrittenCounterIO(RawIOBase):
def __init__(self) -> None:
self.written: int = 0

def write(self, b: Union[bytes, bytearray, memoryview]) -> int:
n = len(b)
self.written += n
return n


mypy 1.17.0 errors with:

$ mypy counter.py
counter.py:8: error: Argument 1 of "write" is incompatible with supertype "_RawIOBase"; supertype defines the argument type as "Buffer" [override]
counter.py:8: note: This violates the Liskov substitution principle
counter.py:8: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides


I know I can use Any but that's annoyingly broad. Is there any solution other than Any or # type: ignore[override]?

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