'A L{ftp.DTPFactory} instance\'s C{buildProtocol} method can be used once
to create a L{ftp.DTP} instance.'

def test_buildProtocolOnce(self):

    protocol = self.factory.buildProtocol(None)
    self.assertIsInstance(protocol, ftp.DTP)
    self.assertIsNone(self.factory.buildProtocol(None))
