def do_RETR(self, i):
        """
        Handle a RETR command.

        @type i: L{bytes}
        @param i: A 1-based message index.

        @rtype: L{Deferred}
        @return: A deferred which triggers after the response to the RETR
            command has been issued.
        """
        return self._sendMessageContent(
            i,
            lambda fp: fp,
            lambda size: "%d" % (size,))

        Handle a RETR command.

        @type i: L{bytes}
        @param i: A 1-based message index.

        @rtype: L{Deferred}
        @return: A deferred which triggers after the response to the RETR
            command has been issued.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/pop3.py