Missing imports for os and Union. Otherwise looks good to me, although personally I'd have left the Optional in the arguments and only put the differing types in _DirT.

oops fixed, I'm spoiled by pre-commit 🙃

urgh, AnyStr is a type variable making this more difficult -- I think I need to duplicate all the signatures? is there a less wacky way to do what I want here?

hmmm now we're failing pytype 🤔

The reason for the pytype failure is that pytype looks for the presence of type variables defined in the same pyi file to figure out whether an alias is generic. If you define _DirT as:

from typing import TypeVar
_T = TypeVar("_T")
if sys.version_info >= (3, 6):
    _DirT = Union[_T, os.PathLike[_T]]
else:
    _DirT = Union[_T]

it should work.

The reason for the pytype failure is that pytype looks for the presence of type variables defined in the same pyi file to figure out whether an alias is generic. If you define _DirT as:

from typing import TypeVar
_T = TypeVar("_T")
if sys.version_info >= (3, 6):
    _DirT = Union[_T, os.PathLike[_T]]
else:
    _DirT = Union[_T]

it should work.

this has different semantics than AnyStr

Everywhere you're using _DirT, it's _DirT[AnyStr], which should replace the _T with AnyStr, unless I've misunderstood how generic type aliases work.

And you can always just copy the definition of AnyStr for _T.

This seems to cause a regression on Python 3.5; see #3449.

$ echo 'import tempfile; tempfile.mkstemp(dir="/var/tmp")' | mypy /dev/stdin
dev/stdin:1: error: Argument "dir" to "mkstemp" has incompatible type "str"; expected "None"
Found 1 error in 1 file (checked 1 source file)