shutil.copyfileobj stopped working in 1.2.0

Bug Report

MyPy 1.2.0 doesn't like shutil.copyfileobj anymore, with "can't infer type argument 1".

To Reproduce

import shutil
import typing

stdin = typing.cast(typing.IO[bytes], None)
with open("random_file.txt", "rb") as from_file:
    shutil.copyfileobj(from_file, stdin)

Expected Behavior

$ pipx run mypy==1.1.1 --strict tmp.py
Success: no issues found in 1 source file

https://mypy-play.net/?mypy=1.1.1&python=3.11&gist=7f9fbe8e5df5f52e5c639e30671eb208

Actual Behavior

$ pipx run mypy==1.2.0 --strict tmp.py
tmp.py:6: error: Cannot infer type argument 1 of "copyfileobj"  [misc]
Found 1 error in 1 file (checked 1 source file)

https://mypy-play.net/?mypy=latest&python=3.11&gist=dbc914004646a0c0ad90926c501d1daa

Your Environment

Mypy version used: 1.1.1 & 1.2.0
Mypy command-line flags: (Seen in playground with nothing)
Mypy configuration options from mypy.ini (and other config files): (Seen in playground with nothing)
Python version used: 3.11 and others.

Yeah, I ran into this too. Seems triggered by change in typeshed: https://github.com/python/mypy/pull/14902/files#diff-363460977156fcfda748f21565484fe1d5862edf2823e9784cbf34d1e52ff2f2

Should I open an issue over there, then?

I think this is actually a mypy bug, though one that was exposed through the typeshed change. It would help to try to create a reproducer that doesn't rely on the stdlib (basically, copy-paste in the definitions of IO, open, and copyfileobj, and simplify them as much as possible).

It looks like #14975 fixes this false positive, which strongly implies to me that this is the same bug as #14943

Here's a minimal-ish repro that doesn't rely on typeshed's stdlib stubs:

from typing import AnyStr, Generic, Protocol, TypeVar, overload

T_co = TypeVar("T_co", covariant=True)
T_contra = TypeVar("T_contra", contravariant=True)

class SupportsRead(Generic[T_co]):
    def read(self) -> T_co: ...

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

def copyfileobj(fsrc: SupportsRead[AnyStr], fdst: SupportsWrite[AnyStr]) -> None: ...

class WriteToMe(Generic[AnyStr]):
    @overload
    def write(self: WriteToMe[str], s: str) -> int: ...
    @overload
    def write(self: WriteToMe[bytes], s: bytes) -> int: ...

class WriteToMeOrReadFromMe(WriteToMe[AnyStr], SupportsRead[AnyStr]): ...

copyfileobj(WriteToMeOrReadFromMe[bytes](), WriteToMe[bytes]())

https://mypy-play.net/?mypy=latest&python=3.11&gist=7443887f86538e58f87bc9b6f1957ce5

It does indeed end up looking very similar to #14943.

ignore mypy lint due to python/mypy#15031

workaround python/mypy#15031

chore(deps-dev): bump mypy from 1.1.1 to 1.4.1

Filter overload items based on self type during type inference (pytho…

Thanks! Type ignore removed in pypa/cibuildwheel#2054.

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shutil.copyfileobj stopped working in 1.2.0 #15031

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shutil.copyfileobj stopped working in 1.2.0 #15031

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hauntsaninja commented on Apr 11, 2023

henryiii commented on Apr 11, 2023

JelleZijlstra commented on Apr 11, 2023

AlexWaygood commented on Apr 11, 2023

AlexWaygood commented on Apr 11, 2023

henryiii commented on Oct 21, 2024

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