>>> list(Register._module_parents('a.b'))
        ['a.b', 'a', '']

def _module_parents(module_name):
        '''
        >>> list(Register._module_parents('a.b'))
        ['a.b', 'a', '']
        '''
        spl = module_name.split('.')
        for i in range(len(spl), 0, -1):
            yield '.'.join(spl[0:i])
        if module_name:
            yield ''