When I run mypy --python-version 3.6 on the following code:

from typing import AnyStr
def f(x: AnyStr) -> bytes:
    return x.encode("ascii") if isinstance(x, str) else x

I get:

test.py:3: error: "bytes" has no attribute "encode"; maybe "decode"?

But as far as I can tell, this is backwards! x.encode("ascii") is only called when isinstance(x, str), not in the else case (where x is a bytes object).

This is with the latest mypy==0.530