From https://github.com/testing-cabal/mock/issues/243

from unittest import mock
mmock = mock.MagicMock()
mmock.foobar("baz")
mmock.assert_has_calls([])  # No exception raised. Why?mmock.assert_has_calls(['x'])  # Exception raised as expected.

---

Traceback (most recent call last):
  File "tt.py", line 7, in <module>
    mmock.assert_has_calls(['x'])  # Exception raised as expected.
  File "/home/robertc/work/cpython/Lib/unittest/mock.py", line 824, in assert_has_calls
    ) from cause
AssertionError: Calls not found.
Expected: ['x']
Actual: [call.foobar('baz')]