So, I know that all three of the usage cases in that chunk of code raise the warnings, but if one of them didn't, is this test still accurate?  Put another way, does <code>len(w)</code> increase with each iteration, even if a warning isn't raised?  If it doesn't, then <code>assert_true(w[-1].category == DeprecationWarning)</code> wouldn't work either, correct?  Would it be better to do:

    for setno in ['Left Auditory', 0, None]:
        with warnings.catch_warnings(record=True) as w:
            warnings.simplefilter('always')
            read_evoked(fname, setno=setno)
            assert_true(w.category == DeprecationWarning)

It seems to me that, at least, this would eliminate the need for checking the length (assuming it's not totally incorrect for some reason to which I'm not savvy).