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.