def sendShort(self, cmd, args):
        """
        Send a POP3 command to which a short response is expected.

        Block all further commands from being sent until the response is
        received.  Transition the state to SHORT.

        @type cmd: L{bytes}
        @param cmd: A POP3 command.

        @type args: L{bytes}
        @param args: The command arguments.

        @rtype: L{Deferred <defer.Deferred>} which successfully fires with
            L{bytes} or fails with L{ServerErrorResponse}
        @return: A deferred which fires when the entire response is received.
            On an OK response, it returns the response from the server minus
            the status indicator.  On an ERR response, it issues a server
            error response failure with the response from the server minus the
            status indicator.
        """
        d = self._blocked(self.sendShort, cmd, args)
        if d is not None:
            return d

        if args:
            self.sendLine(cmd + b' ' + args)
        else:
            self.sendLine(cmd)
        self.state = 'SHORT'
        self._waiting = defer.Deferred()
        return self._waiting

        Send a POP3 command to which a short response is expected.

        Block all further commands from being sent until the response is
        received.  Transition the state to SHORT.

        @type cmd: L{bytes}
        @param cmd: A POP3 command.

        @type args: L{bytes}
        @param args: The command arguments.

        @rtype: L{Deferred <defer.Deferred>} which successfully fires with
            L{bytes} or fails with L{ServerErrorResponse}
        @return: A deferred which fires when the entire response is received.
            On an OK response, it returns the response from the server minus
            the status indicator.  On an ERR response, it issues a server
            error response failure with the response from the server minus the
            status indicator.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/pop3client.py