a generator method which walks the file structure of the dropbox collection

def _walk(self, root_path='', root_id=''):
        
        ''' a generator method which walks the file structure of the dropbox collection '''
        
        title = '%s._walk' % self.__class__.__name__
        
        if root_id:
            pass
        elif root_path:
            root_id, root_parent = self._get_id(root_path)
        for file_id, name, mimetype in self._list_directory(root_id):
            file_path = os.path.join(root_path, name)
            if mimetype == 'application/vnd.google-apps.folder':
                for path, id in self._walk(root_path=file_path, root_id=file_id):
                    yield path, id
            else:
                yield file_path, file_id