In issue http://bugs.python.org/issue21238 a feature was introduced to help prevent false-positive test cases by throwing an AttributeError whenever a non-existent method prefixed with "assert_" is used on a mock object.

I had mock objects with legitimate "assert_*" methods that had been working for some time.  My tests included calls like:

my_mock.assert_in_status.assert_called_once_with(...)

My tests started failing unexpectedly and it took me HOURS to figure out that I had a new mock version installed.  Those hours could have been turned into seconds my simply giving a better error message, something like:

AttributeError: you used "assert_in_status" but that method is not a valid Mock assert method.  Please check your spelling.  If this was not a typing mistake, you can use the `unsafe` keyword argument to the Mock instance to turn this validation check off.  See https://mock-docs...org/ for more details.

How were you setting/introducing your assertions on the mocks? e.g. could you supply a small sample script showing what used to work? Thanks.

Old functionality:

(temp)rsyring@loftex:~/projects/hllapi-src$ python
Python 2.7.6 (default, Jun 22 2015, 17:58:13) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mock
>>> mock.__version__
'1.0.0'
>>> m = mock.Mock()
>>> m.assert_screen_status.call_count
0

New functionality:

(temp)rsyring@loftex:~/projects/hllapi-src$ python
Python 2.7.6 (default, Jun 22 2015, 17:58:13) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mock
>>> mock.__version__
'1.1.0'
>>> m = mock.Mock()
>>> m.assert_screen_status.call_count
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/rsyring/.virtualenvs/temp/local/lib/python2.7/site-packages/mock/mock.py", line 714, in __getattr__
    raise AttributeError(name)
AttributeError: assert_screen_status

In my case, the objects I was patching had a legit method like assert_screen_status().  But, after upgrading Mock, the use of those methods started throwing AttributeError's even though it was very obvious that a) the methods existed on the real objects and b) Mock is supposed to let me call anything on I want.  As I said previously, this was a VERY confusing situation to figure out.

@Randy - ok thanks. So, please do improve the prose in the error message, should be a very straight forward patch.

I've created a PR for this issue.

New changeset b9b08cd948de97d756a199b60becce8397a8c882 by Kushal Das (Zackery Spytz) in branch 'master':
bpo-24758: Improve the error msg for unittest.mock.Mock()'s unsafe mode (#12991)
https://github.com/python/cpython/commit/b9b08cd948de97d756a199b60becce8397a8c882

Closing this as resolved since PR was merged. Thanks Zackery for the PR.