I think "Actual" is more consistent with other error messages like assert_has_calls. I haven't seen usage of observed or received in error messages and hence my opinion on using expected and actual though they are not visually aligned. I don't have a strong opinion on this. I will leave it to the maintainer who wants to merge this change and I am fine as long as the calls are represented on separate lines which is a good improvement over current format.

>>> m.assert_has_calls(mock.call(1))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", line 857, in assert_has_calls
    raise AssertionError(
AssertionError: Calls not found.
Expected: ['', (1,), {}]
Actual: [call(1), call(2)].