How to specify constants of type NoneType?

PEP 484 says "By default, None is an invalid value for any type". However, in the Python standard library there are class-level and module-level constants that are legitimately None.

Examples:

  formatter.py.AS_IS
  hmac.digest_size
  cgi.FieldStorage.FieldStorageClass
  copy.PyStringMap

Should we allow these to be declared using e.g. something like

  AS_IS = Undefined(NoneType)

?

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.

I was trying to avoid proposing that requires mypy changes, since it looks like mypy treats the type `None` quite special. But sure, we'll add that to the list of possibilities. @JukkaL, thoughts?

Describe type vars with co/contra variance. Fixes python#2.

Merge pull request python#2 from python/master