How do I add a type hint for a WriteableBuffer parameter?

I'm trying to add a parameter type to the readinto() method declared in a custom class that derives from RawIOBase, like this:

from io import RawIOBase

class Reader(RawIOBase):
    def readinto(self, buf: bytearray) -> int:
        pass  # actual implementation omitted

But pyright complains:

io.py:6:9 - error: Method "readinto" overrides class "_RawIOBase" in an incompatible manner
    Parameter 2 type mismatch: base parameter is type "WriteableBuffer", override parameter is type "bytearray"
      "Buffer" is not assignable to "bytearray" (reportIncompatibleMethodOverride)
1 error, 0 warnings, 0 informations

How do I fix this? Note: I know I can remove the type hint entirely. I want to assign it the correct type instead.

I'm using Python 3.13.3 and pyright 1.1.400.

Solution

You need to use the same type definition as your base. You can use the same type alias here by importing it from _typeshed package, provided you put it under a TYPE_CHECKING guard:

from io import RawIOBase
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from _typeshed import MaybeNone, WriteableBuffer

class Reader(RawIOBase):
    def readinto(self, buf: WriteableBuffer) -> int | MaybeNone:
        ...

You can count on the _typeshed package existing when running under a type checker, because the type checker brings along that package. This is how pyright knows what the signature for RawIOBase.readinto() is, for example. I added MaybeNone to the above for the same reason, because that's also part of the documented signature.

Alternatively, because WriteableBuffer is just a type alias for collections.abc.Buffer, the following also works (replacing MaybeNone with a redefinition):

from collections.abc import Buffer
from io import RawIOBase
from typing import Any, TypeAlias

type MaybeNone = Any

class Reader(RawIOBase):
    def readinto(self, buf: Buffer) -> int | MaybeNone:
        ...

See The Any Trick for an explanation as to why MaybeNone is an alias for Any.