code: def assert_is_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 not in", extra)
docs: Raises an AssertionError if substring is not a substring of subject.