code: def ls(self):
          
          if self.isfile():
              raise NotDirectoryError('Cannot ls() on non-directory node: {path}'.format(path=self._pyerarchy_path))
  
          return os.listdir(self._pyerarchy_path)
docs: List the children entities of the directory.
  
          Raises exception if the object is a file.
  
          :return: