The variables could be of their concrete type or of an optional concrete type depending on whether it is desirable to have the check for None enabled for them.

For example, a PEP 484 compatible definition of hmac.digest_size could look like this:

digest_size = ...  # type: int

or like this:

digest_size = ... # type: Optional[int]

Maybe one of the following should be declared best practice?

foo = None  # type: object
foo = None  # type: Optional[object]

All of these work today with mypy.

What about

foo = None  # type: None

It doesn't work in mypy yet, but mypy's error message sounds like it could, in the future.

This is almost(ish :-) over. With --strict-optional, mypy does not complain about

x = None # type: None

We'll just have to finish for the strict None checking flag in mypy to become the default.

x = None # type: None works fine in mypy with python/mypy#3024 merged.

I'm submitting a PR to fix the specific objects @matthiaskramm mentioned.