Could you add a test to verify that custom actions are still getting the converted values passed to their __call__? I suspect this may not be happening under the current patch - if that's the case, you may also need to add conversions in _get_values, where the lines look like "value = action.default".

Also, "action.default == getattr(namespace, action.dest)" should probably use "is" instead of "==".

Other than that, the patch looks okay.