According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

Hm. Okay. So the test suite seems to be correct: Python 3.7's typing module does indeed not declare TextIO.reconfigure() and TextIO.write_through. However, they are present in the actual sys.stdin/stdout/stderr objects. So... what do we do here? And okay, fine, "do nothing and add type: ignore on the sys.stdin.reconfgure() line" is an acceptable answer.

This was already "fixed" in #4035, but that fix doesn't work because sys.stdin is typed as TextIO, not as TextIOWrapper which is the class that provides reconfigure(). Changing sys.stdin to be a TextIOWrapper isn't great either, because then you can't do sys.stdin = io.StringIO().

A couple things we could do instead:

• Use sys.__stdin__.recoonfigure(). The __stdin__ attribute isn't meant to be reassigned to a StringIO or whatever other file-like object, so it can be typed as a TextIOWrapper, and it's already typed that way. If sys.stdin hasn't been reassigned, __stdin__ and stdin refer to the same object.
• Change the type of stdin to be TextIOWrapper | Any. This means your code must work if stdin is a TextIOWrapper, but it could technically be something else too, and assigning something else to it is not an error.

You can also use isinstance() like in #4035.

typeshed's TextIO type follows the implementation of TextIO in Python's standard library. The latter doesn't have write_through() or reconfigure(). Also, considering the pseudo-protocol nature of TextIO, it's difficult to add items to it, even in the standard library.

To be clear, here's how to do this with isinstance:

if isinstance(sys.stdout, io.TextIOWrapper) and sys.version_info >= (3, 7):
    sys.stdout.reconfigure(encoding="UTF-8")

Thanks, that helps a lot! And yes, I get it, the I/O types are, hm, complicated. So yeah, thanks, and sorry for the noise!