code: def assert_is_not_substring(substring, subject, message=None, extra=None):
      
      assert (
          (subject is not None)
          and (substring is not None)
          and (subject.find(substring) == -1)
      ), _assert_fail_message(message, substring, subject, "is in", extra)
docs: Raises an AssertionError if substring is a substring of subject.