`BufferedWriter` is not compatible with `SupportsWrite[AnyStr]`

[tmke8]

Describe the bug
Sorry for the rambling title – I couldn't think of a good description of this:

import shutil
from typing import TYPE_CHECKING, AnyStr
from io import BufferedWriter

if TYPE_CHECKING:
    from _typeshed import SupportsRead, SupportsWrite


def f(s: SupportsRead[AnyStr], t: SupportsWrite[AnyStr]) -> None:  # with TypeVar
    shutil.copyfileobj(s, t)


def g(s: SupportsRead[bytes], t: SupportsWrite[bytes]) -> None:  # with concrete type
    shutil.copyfileobj(s, t)


def h(src: SupportsRead[bytes], tgt: BufferedWriter) -> None:
    f(src, tgt)  # E: Argument of type "BufferedWriter" cannot be assigned to parameter "t" of type "SupportsWrite[AnyStr@f]" in function "f"
    g(src, tgt)

Expected behavior
If I can pass tgt succesfully to g, then I don't see why it would fail with f. Clearly, BufferedWriter is compatible with SupportsWrite[bytes].

VS Code extension or command-line
command-line

Additional context
This happened after updating to the most recent version. I believe there were typeshed changes around this code.

[tmke8]

Actually, this is probably a duplicate of #5027.

[erictraut]

I don't think this is related to #5027. It doesn't involve a protocol implementation with a method-local TypeVar.

This looks like a problem with recent changes in typeshed.

Let's look more closely at your example. Function f passes an argument of type SupportsRead[bytes] for the first parameter. That means the AnyStr@f type variable will be constrained to bytes, which means that the second argument must be compatible with the type SupportsWrite[bytes], which is a protocol that has a method write. Once specialized, this method has the type def write(self, __s: bytes) -> object: .... The second argument that you're passing to f is of type BufferedWriter. It also has a method called write, but the type is def write(self, __buffer: ReadableBuffer) -> int: .... The type ReadableBuffer is a protocol that requires the type to have a method called __buffer__, but the bytes class doesn't have any such method. That means the two write methods are incompatible.

I presume this typeshed change was prompted by PEP 688.

I'm tempted to revert the typeshed update until we can determine what's going on here. This is likely to break a lot of code. I just confirmed that it breaks the pandas-stubs project, for example.

@JelleZijlstra, perhaps you can provide some insights here?

[JelleZijlstra]

but the bytes class doesn't have any such method.

We in fact added __buffer__ to bytes across all versions, as discussed in python/typeshed#10224. It's here in typeshed: https://github.com/python/typeshed/blob/786bd22343a66ecdb0eeb3fe053dfb81365608c5/stdlib/builtins.pyi#L713 .

[erictraut]

Ah, I missed that. OK, then this might be a type analysis bug. I'll dig deeper. Thanks for the quick response.

[erictraut]

I've reverted the typeshed changes for the time being and published a new version of pyright (1.1.313) that addresses this regression. I wanted to do this prior to this week's build of pylance (which is done on Wednesday morning's). This will give me time to investigate the underlying cause of this problem prior to next week's build.

[erictraut]

After looking at this more closely, I don't think this is a bug in pyright. Here's a self-contained, minimal code sample that demonstrates the problem:

from typing import AnyStr, Protocol, TypeVar

_T_co = TypeVar("_T_co", covariant=True)
_T_contra = TypeVar("_T_contra", contravariant=True)

class Buffer(Protocol):
    def __buffer__(self, __flags: int) -> memoryview:
        ...

class SupportsRead(Protocol[_T_co]):
    def read(self, __length: int = ...) -> _T_co:
        ...

class SupportsWrite(Protocol[_T_contra]):
    def write(self, __s: _T_contra) -> object:
        ...

class BufferedWriter:
    def write(self, __buffer: Buffer) -> int:
        raise NotImplementedError

def f(s: SupportsRead[AnyStr], t: SupportsWrite[AnyStr]) -> None:
    ...

def h(src: SupportsRead[bytes], tgt: BufferedWriter) -> None:
    f(src, tgt)

The issue is that BufferedWriter is not type compatible with SupportsWrite[AnyStr]. That's because AnyStr can be a str, and that's not compatible with Buffer. Mypy also emits an error for this code.

@JelleZijlstra, I don't fully understand the recent typeshed changes, but they appear to be creating a lot of new errors in existing typed code bases. I presume that you ran mypy_primer with the changes. I'm curious whether that indicated any significant problems for existing code bases? I'm hesitant to upgrade the bundled stubs for pyright until we've worked through some of these issues.

Another issue that seems to be related to this one is in the pandas-stubs project. It expects the type io.BytesIO to be type compatible with WriteBuffer[bytes] | WriteBuffer[str] where WriteBuffer is an internally-defined protocol class.

import io
from typing import Any, Protocol, TypeVar
AnyStr_contra = TypeVar("AnyStr_contra", str, bytes, contravariant=True)

class BaseBuffer(Protocol):
    @property
    def mode(self) -> str: ...
    def seek(self, __offset: int, __whence: int = ...) -> int: ...
    def seekable(self) -> bool: ...
    def tell(self) -> int: ...

class WriteBuffer(BaseBuffer, Protocol[AnyStr_contra]):
    def write(self, __b: AnyStr_contra) -> Any: ...
    def flush(self) -> Any: ...

def test(b: io.BytesIO):
    x: WriteBuffer[bytes] | WriteBuffer[str] = b # Type error with new typeshed

Let me know if you'd like me to open individual issues in the typeshed project to work through these issues (and others I'm seeing).

[JelleZijlstra]

The issue is that BufferedWriter is not type compatible with SupportsWrite[AnyStr]. That's because AnyStr can be a str, and that's not compatible with Buffer. Mypy also emits an error for this code.

I think this should pass: in the call to f(), AnyStr should be solved to bytes, and then BufferedWriter is compatible with SupportsWrite[bytes].

Mypy emits an error here because in its bundled stubs, bytes doesn't yet have a __buffer__ method. However, if you change Buffer to some other method that bytes has and str doesn't, it succeeds: https://mypy-play.net/?mypy=latest&python=3.12&gist=6ade2ba7bf514edbef7369b7dc33f511

Pyright fails on that sample, which I think is a bug.

I'll need some more time to wrap my head around the pandas example, but in general please feel free to open bugs on typeshed.