In the format string for assert_called the evaluation order is incorrect and hence for mock's without name 'None' is printed whereas it should be 'mock' like for other messages. The error message is ("Expected '%s' to have been called." % self._mock_name or 'mock') . Here self._mock_name which is None is applied to form the string and then used with the string 'mock' in or combination. The fix would be to have the evaluation order correct like other error messages. Marking this as newcomer-friendly. Please leave this to new contributors as their 1st PR.

./python.exe
Python 3.9.0a0 (heads/master:f03b4c8a48, Aug 12 2019, 10:04:10)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from unittest.mock import Mock
>>> m = Mock()
>>> m.assert_called_once()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", line 854, in assert_called_once
    raise AssertionError(msg)
AssertionError: Expected 'mock' to have been called once. Called 0 times.
>>> m.assert_called()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", line 844, in assert_called
    raise AssertionError(msg)
AssertionError: Expected 'None' to have been called.


Thanks

New changeset 5f5f11faf9de0d8dcbe1a8a4eb35d2a4232d6eaa by Pablo Galindo (Abraham Toriz Cruz) in branch 'master':
bpo-37828: Fix default mock_name in unittest.mock.assert_called error (GH-16166)
https://github.com/python/cpython/commit/5f5f11faf9de0d8dcbe1a8a4eb35d2a4232d6eaa

New changeset f668d2b775da4bcd07e142c4bc5ebd88165fadf4 by Miss Islington (bot) in branch '3.8':
bpo-37828: Fix default mock_name in unittest.mock.assert_called error (GH-16166)
https://github.com/python/cpython/commit/f668d2b775da4bcd07e142c4bc5ebd88165fadf4

I don't think 3.7 needs a backport of the fix. I am closing this as fixed since all PRs are merged. Please reopen if a backport is needed. Thanks @categulario.