The correct way to annotate a “file type” in Python [duplicate]

In modern versions of Python one can have static type analysis using function annotations, according to PEP 484. This is made easy through the typing module.

Now I’m wondering how I would give a “type hint” towards a “filestream”.

def myfunction(file: FILETYPE):
    pass

with open(fname) as file:
    myfunction(file)

What would I insert as FILETYPE?

Using print(type(file)) returns <class '_io.TextIOWrapper'> which isn’t clear at all.

Isn’t there a generic “file” type?

Answer

You can use typing.IO, typing.TextIO, and typing.BinaryIO to represent different types of I/O streams. To quote the documentation:

сlass typing.IO
class typing.TextIO
class typing.BinaryIO

Generic type IO[AnyStr] and its subclasses TextIO(IO[str]) and BinaryIO(IO[bytes]) represent the types of I/O streams such as returned by open().