Correct type for binary file-like object

Question

I have the following functions defined using Python type-hinting:

from typing import BinaryIO

def do_something(filename: str):
    my_file = open(filename, "rb")
    read_data(my_file)

def read_data(some_binary_readable_thing: BinaryIO):
    pass

However my IDE (PyCharm 2017.2) gives me the following warning on the line I invoke read_file:

Expected type 'BinaryIO', got 'FileIO[bytes]' instead

What is the correct type for me to use here? PEP484 defines the BinaryIO as "a simple subtype of IO[bytes]". Does FileIO not conform to IO?

Answer 1

This looks like a bug in either Pycharm or typing module. From the typing.py module:

class BinaryIO(IO[bytes]):
    """Typed version of the return of open() in binary mode."""
    ...

Also the documentation specifies:

These represent the types of I/O streams such as returned by open().

So it should work as stated. For now, a workaround would be to explicitly use FileIO.

from io import FileIO

def do_something(filename: str):
    my_file = open(filename, "rb")
    read_data(my_file)

def read_data(some_binary_readable_thing: FileIO[bytes]):
    pass