Since the classes are all subclasses of `_MyBase`, they can be accessed via `_MyBase.__subclasses__()` after they have been imported:

    for (module_loader, name, ispkg) in pkgutil.iter_modules([pkg_dir]):
        importlib.import_module('.' + name, __package__)
        
    all_my_base_classes = {cls.__name__: cls for cls in base._MyBase.__subclasses__()}

For importing the modules, I followed [the advice of Nihathrael][1].


  [1]: http://codereview.stackexchange.com/a/70280/10916