When I call unittest.TestCase.assertEqual(a, b) on e.g. two unequal dictionaries, I get a nice diff pointing me to the differences.

>>> class A(unittest.TestCase):
...     def test_foo(self):
...         self.assertEqual(dict(foo='bar', zab='zar'), dict(foo='bar', zab='zab'))
>>> unittest.main()
======================================================================
FAIL: test_foo (__main__.A)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "<stdin>", line 3, in test_foo
AssertionError: {'foo': 'bar', 'zab': 'zar'} != {'foo': 'bar', 'zab': 'zab'}
- {'foo': 'bar', 'zab': 'zar'}
?                          ^

+ {'foo': 'bar', 'zab': 'zab'}
?                          ^


----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

But when unittest.mock.Mock.assert_called_with fails, I don't get this nice diff output.

This would be very helpful in my present case (asserting that a function with many keyword arguments is called correctly).