It seems what you're saying is that the ArgumentTypeError class should not be public, but being able to raise is should be public. If that's the case, I think it would be more clear to have an argparse.raiseArgumentTypeError method and document when it should be used.

If such classes are meant to be private, why not prepend their names with an underscore and remove them from the __all__ list? (I thought a leading underscore meant that a module level variable was private to that module.)