Just got bitten by this again. Context: I have a protocol which is sending JSON datagrams over the wire and I'm checking the sent data. I can't exactly check the JSON-encoded content since dict ordering is undefined. So right now I have to write:

    self.assertEqual(q.transport.send.call_count, 1)
    (dgram, target), _ = q.transport.send.call_args
    self.assertEqual(target, (PEER, PORT))
    self.assertEqual(json.loads(dgram), { ... })

Clumsy, clumsy (note that I am too lazy to add a check for the kwargs).

Would be much better if I could simply write:

    dgram, = q.tranport.assert_called_once_with(mock.ANY, (PEER, PORT))
    self.assertEqual(json.loads(dgram), { ... })