Previous question: How do I add a type hint for a WriteableBuffer parameter? I'm trying to implement the readinto() method of a RawIOBase subclass with correct type hints. The version I have so far looks like this: from io import RawIOBase from typing import TYPE_CHECKING if TYPE_CHECKING: from _typeshed import WriteableBuffer class Reader(RawIOBase): def readinto(self, buf: WriteableBuffer): n = len(buf) # <-- this is not allowed # ... do something with n ... return super().readinto(buf) pyright complains about calling len() on buf: io.py:9:19 - error: Argument of type "WriteableBuffer" cannot be assigned to parameter "obj" of type "Sized" in function "len" "Buffer" is incompatible with protocol "Sized" "__len__" is not present (reportArgumentType) 1 error, 0 warnings, 0 informations If I understand correctly WriteableBuffer is defined here, as: WriteableBuffer: TypeAlias = bytearray | memoryview | array.array[Any] | mmap.mmap | ctypes._CData # stable All of these definitely have a defined length, except ctypes._CData, where only some subclasses define __len__ (e.g., arrays do, primitive values like int don't). But it seems the conclusion is that a WriteableBuffer just doesn't have a length, which is weird, because how should you write to it if you don't know its size? Is there a way to fix this code? Or should I just add a # type: ignore to the lines involving len(buf)? Continue reading...
