from unittest.mock import Mock
class Foo:
    def assert_something(self):
         pass

m = Mock(spec=Foo)
m.assert_something()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/cklein/github/cklein/cpython/Lib/unittest/mock.py", line 657, in __getattr__
    raise AttributeError(
AttributeError: 'assert_something' is not a valid assertion. Use a spec for the mock if 'assert_something' is meant to be an attribute.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/unittest/mock.py", line 635, in __getattr__
    raise AttributeError("Attributes cannot start with 'assert' "
AttributeError: Attributes cannot start with 'assert' or 'assret'