def longest(words):
    try:    # attempt the for loop and see if it goes awry
        empty = True
        for w in words:    # this might cause a TypeError
            if not isinstance(w, str):
                raise TypeError('elements must be strings')
            empty = False
        if empty:
            raise ValueError('sequence must be nonempty')

    # technically, we should have done the real work above as well,
    # as we may have just stripped all the data out of the iterable