Bare Union, Callable and Tuple should not be classes

[gvanrossum]

There is no reason for "bare" Union, Callable or Tuple (i.e. without [...]) to be a class. You shouldn't ever write isinstance(x, Union) for example, and in their bare form these are not acceptable as annotations either.

[agronholm]

I've been using Callable like that. What's wrong with doing so? Should I import Callable from collections.abc then? Or always use Callable[..., Any]?

[gvanrossum]

Hm, that's a good point. Pretty much anything you can do with collections.abc.X should be possible with typing.X. (This does not apply to things that aren't defined in collection.abc though, so this is not the case for Union, Tuple or List etc.)