How to correctly override BinaryIO.write?

[rpmrmartin]

I have written a BinaryIO decorator, but I'm having trouble adding type hints to the write method that will pass Pyright without using Any. Here's my first attempt:

# pyright: strict

from typing import BinaryIO

class Decorator(BinaryIO):

	def __init__(self, file: BinaryIO) -> None:
		self._file = file
    
	def write(self, s: bytes) -> int:
		return self._file.write(s)

This passed Pyright 1.1.311, but fails 1.1.313 with the following error:

/app/temp.py
  /app/temp.py:10:6 - error: Method "write" overrides class "IO" in an incompatible manner
    Parameter 2 type mismatch: base parameter is type "str", override parameter is type "bytes"
    Return type mismatch: base method returns type "int", override returns type "None"
      "str" is incompatible with "bytes"
      Type "None" cannot be assigned to type "int" (reportIncompatibleMethodOverride)
1 error, 0 warnings, 0 informations

Looking at typeshed, I think Pyright is complaining about the method not being compatible with the following overload:

@overload
def write(self: IO[str], __s: str) -> int: ...

But self is not an IO[str], so I don't think that overload applies. Is this a bug or do I misunderstand?

I also tried copying the annotations from typeshed as closely as I could. I checked this with Pyright built from d20a39c so it wouldn't complain about importing Buffer:

# pyright: strict

from typing import Any, AnyStr, BinaryIO, overload

from typing_extensions import Buffer

class Decorator(BinaryIO):

	def __init__(self, file: BinaryIO) -> None:
		self._file = file

	@overload
	def write(self, s: str) -> int: ...

	@overload
	def write(self, s: Buffer) -> int: ...

	@overload
	def write(self, s: AnyStr) -> int: ...
	
	def write(self, s: Any) -> int:
		return self._file.write(s)

For this, Pyright generates the following error:

/app/temp.py
  /app/temp.py:21:6 - error: Method "write" overrides class "IO" in an incompatible manner
    Overloads for override method must be in the same order as the base method (reportIncompatibleMethodOverride)
1 error, 0 warnings, 0 informations

The overloads are in the same order as the base method, so I don't understand what Pyright is trying to tell me here, unless the overloads do not match the base method closely enough. If this is not a bug, could someone explain why Pyright is generating this error?

[erictraut]

There is a bug in the logic for overloaded overrides, which I added recently. I'll fix this in the next release.

That said, to avoid the reportIncompatibleMethodOverride error, you'll need to copy the overload signatures from typeshed exactly. That includes the use of a positional-only parameter __s rather than a named parameter s. It also includes type annotations for the self parameter on the first two overloads.

class Decorator(BinaryIO):
    def __init__(self, file: BinaryIO) -> None:
        self._file = file

    @overload
    def write(self: IO[str], s: str, /) -> int:
        ...

    @overload
    def write(self: IO[bytes], s: ReadableBuffer, /) -> int:
        ...

    @overload
    def write(self, s: AnyStr, /) -> int:
        ...

    def write(self, s: Any, /) -> int:
        return self._file.write(s)

[rpmrmartin]

Thank you for your help. I cannot copy the overload signatures exactly because ReadableBuffer comes from an internal _typeshed package and typing_extensions has no equivalent. ReadableBuffer is an alias of Buffer, but using Buffer in its place is giving me the same reportIncompatibleMethodOverride error. Do you have any suggestions on how to overcome this?

[erictraut]

You can use Buffer in place of ReadableBuffer. You won't be able to eliminate the reportIncompatibleMethodOverride until the next release of pyright, which addresses the bug. I publish a new version of pyright weekly on Tuesdays, so you won't have to wait long.

[rpmrmartin]

Thanks again!