code: def delete(self):
          
          if not self._deleted:
              if self.api._req_lixian_task_del(self):
                  self._deleted = True
                  return True
          raise TaskError('This task is already deleted.')
docs: Delete task (does not influence its corresponding directory)
  
          :return: whether deletion is successful
          :raise: :class:`.TaskError` if the task is already deleted