def test_fetchBody(self):
        """
        L{IMAP4Client.fetchBody} sends the I{FETCH BODY} command and returns a
        L{Deferred} which fires with a C{dict} mapping message sequence numbers
        to C{dict}s mapping C{'RFC822.TEXT'} to that message's body as given in
        the server's response.
        """
        d = self.client.fetchBody('3')
        self.assertEqual(
            self.transport.value(), b'0001 FETCH 3 (RFC822.TEXT)\r\n')
        self.client.lineReceived(b'* 3 FETCH (RFC822.TEXT "Message text")')
        self.client.lineReceived(b'0001 OK FETCH completed')
        self.assertEqual(
            self.successResultOf(d),
            {3: {'RFC822.TEXT': 'Message text'}})

        L{IMAP4Client.fetchBody} sends the I{FETCH BODY} command and returns a
        L{Deferred} which fires with a C{dict} mapping message sequence numbers
        to C{dict}s mapping C{'RFC822.TEXT'} to that message's body as given in
        the server's response.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_imap.py