This example shows the behavior: from warnings import catch_warnings def test(): with catch_warnings(True) as w: assert str(w.message) == "foo", "%r != %r" % (w.message, "foo") test() This fails with an IndexError from the `w.message`. That's a bit surprising, and since this is mostly an API useful for testing, it'd be much better if it had a well-defined, documented (ie, stable and likely to continue working in the next release of Python) error mode.