How to properly type hint IO[bytest]

[randolf-scholz]

I have the following code snippet

from pathlib import Path
from typing import IO
from io import IOBase

def read_file(file: str | Path | IO[bytes]):
    ...
    file.seek(0, 0) if isinstance(file, IOBase) else None

And mypy complains:

error: Subclass of "IO[bytes]" and "IOBase" cannot exist: would have incompatible method signatures  [unreachable]

I thought one was supposed to use typing.IO for type hinting here?

[srittau]

IO classes are weird. I would recommend to use hasattr(file, "seek") instead of isinstance, although all IO classes are guaranteed to have a seek() method.