Note that there is nothing stopping you using the mock.ANY and assert_called_once_with style assert currently. You're making your assert more clumsy than it needs to be.

With my proposal you could write:

    q.tranport.assert_called_once_with(mock.ANY, (PEER, PORT))
    dgram = q.tranport.call_args.args[0]
    self.assertEqual(json.loads(dgram),expected)

You could currently be doing:

    q.tranport.assert_called_once_with(mock.ANY, (PEER, PORT))
    dgram = q.tranport.call_args[0][0]
    self.assertEqual(json.loads(dgram), expected)