def test_buildProtocolOnce(self):
        """
        A L{ftp.DTPFactory} instance's C{buildProtocol} method can be used once
        to create a L{ftp.DTP} instance.
        """
        protocol = self.factory.buildProtocol(None)
        self.assertIsInstance(protocol, ftp.DTP)

        # A subsequent call returns None.
        self.assertIsNone(self.factory.buildProtocol(None))

        A L{ftp.DTPFactory} instance's C{buildProtocol} method can be used once
        to create a L{ftp.DTP} instance.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_ftp.py