<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _depth_event(self, msg):
 """Handle a depth event :param msg: :return: """


        if 'e' in msg and msg['e'] == 'error':
            # close the socket
            self.close()

            # notify the user by returning a None value
            if self._callback:
                self._callback(None)

        if self._last_update_id is None:
            # Initial depth snapshot fetch not yet performed, buffer messages
            self._depth_message_buffer.append(msg)
        else:
            self._process_depth_message(msg)