File streams: "BufferedWriter" has incompatible type "BinaryIO"; expected "RawIOBase"

[h6197627]

Bug Report
This code

import io

bf = open('binary.file', 'wb', buffering=0)
print(isinstance(bf, io.RawIOBase)) # > True
bw = io.BufferedWriter(bf)

throws:

test.py:5: error: Argument 1 to "BufferedWriter" has incompatible type "BinaryIO"; expected "RawIOBase"

Your Environment

• Mypy version used: 0.812
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.6.9
• Operating system and version: Ubuntu 18.04

[JelleZijlstra]

I fixed this in typeshed a while ago with a serious of overloads for open that return precise types. Maybe that didn't make it into the last mypy release? It should pick the overload at https://github.com/python/typeshed/blob/master/stdlib/builtins.pyi#L1084.

[h6197627]

Just checked, I have

# Unbuffered binary mode: returns a FileIO
@overload
def open(
    file: _OpenFile,
    mode: OpenBinaryMode,
    buffering: Literal[0],
    encoding: None = ...,
    errors: None = ...,
    newline: None = ...,
    closefd: bool = ...,
    opener: Optional[_Opener] = ...,
) -> FileIO: ...

in /usr/local/lib/python3.6/dist-packages/mypy/typeshed/stdlib/3/builtins.pyi

[srittau]

I can confirm. With mypy 0.812 it doesn't pick up the correct overload, although it exists in the bundled typeshed and it looks like it should pick it.

[JelleZijlstra]

It's actually because mypy still has a custom plugin for open that produces less precise types. #9275 removes it, but hasn't been merged yet.

[h6197627]

Hi,
any progress on this issue? :)

[h6197627]

Fixed with #9275

[lifepillar]

I can still reproduce it with mypy 0.931:

$ mypy test.py
foo.py:5:24: error: Argument 1 to "BufferedWriter" has incompatible type "BinaryIO"; expected "RawIOBase"
Found 1 error in 1 file (checked 1 source file)
$ mypy --version
mypy 0.931

Edit: …but not with mypy 0.961. Ok then, I guess.