Pyright: `open(filename, "rb")` recognised as IO[Any] instead of BinaryIO.

Created on 13 Jun 2020 · 5Comments · Source: microsoft/pyright

Describe the bug
When passing file-like object opened in a binary mode to a function that accepts BinaryIO object, pyright throws an error:

Argument of type "IO[Any]" cannot be assigned to parameter "file" in function "foo" of type "BinaryIO"
  "IO[Any]" is incompatible with "BinaryIO" (reportGeneralTypeIssues)

To Reproduce
Steps to reproduce the behavior.

Consider this piece of code:

from typing import BinaryIO


def foo(file: BinaryIO) -> bytes:
    return file.read()


with open("test.txt", "rb") as f:
    foo(f)

Both in VS Code extension and CLI there is an error mentioned above.

Expected behavior
The type should be recognised correctly as BinaryIO.

Additional context

VS Code extension: - 1.1.42
- settings:
  
  json { "pyright.typeCheckingMode": "strict", "pyright.useLibraryCodeForTypes": true }
Python: 3.8.2
Platform: Linux, Ubuntu 19.10

addressed in next version enhancement request

Source

pawelrubin

All 5 comments

Pyright is following the type information provided in the stdlib typeshed stub. The type information for the "open" method is:

def open(file: Union[str, bytes, int, _PathLike[Any]], mode: str = ..., buffering: int = ..., encoding: Optional[str] = ..., errors: Optional[str] = ..., newline: Optional[str] = ..., closefd: bool = ..., opener: Optional[Callable[[str, int], int]] = ...) -> IO[Any]: ...

As you can see, the return type is IO[Any].

Python 3.8 introduced support for "Literal", so it would be possible to update the typeshed stub file to include @overload versions of the open method that return different types based on the literal value passed to the mode parameter. That change would need to be made in the (typeshed repo)[https://github.com/python/typeshed]. You're welcome to submit a change or file a bug in that project. If you get that fixed, let me know, and I'll pull an updated version of the typeshed stubs into Pyright.

In the meantime, you can work around the issue by adding an assert to your code: