Using hashlib.file_digest with a file object obtained via pathlib.Path.open failed to typecheck with a protocol compatibility error:

src/create_bundles.py:223: error: Argument 1 to "file_digest" has incompatible type "FileIO"; expected "_BytesIOLike | _FileDigestFileObj"  [arg-type]
src/create_bundles.py:223: note: Following member(s) of "FileIO" have conflicts:
src/create_bundles.py:223: note:     Expected:
src/create_bundles.py:223: note:         def readinto(self, bytearray, /) -> int
src/create_bundles.py:223: note:     Got:
src/create_bundles.py:223: note:         def readinto(self, Buffer, /) -> int | None

I'm reasonably sure this is a typeshed issue rather than a mypy issue.

My first thought was that this looks like an internal inconsistency in typeshed, as _FileDigestFileObj is declared specifically with bytearray rather than the more general WriteableBuffer (hence filing the issue here).

However, my second thought was that this signature declaration is presumably as it is because file_digest will specifically pass a bytearray instance to readinto, and bytearray is compatible with Buffer, so mypy shouldn't be complaining about that. That means the conflict is presumably on the return type rather than on parameter types.

The declared type signature nominally indicates that file_digest can't cope with readinto returning None, and mypy is correctly flagging that as inconsistent with the way typeshed declares the file and socket IO types (allowing them to return None here).

Searching the typeshed code suggests this particular inconsistency exists across several different IO consumer type declarations: https://github.com/search?q=repo%3Apython%2Ftypeshed%20readinto&type=code