def longest(words):
    # attempt a for loop over the parameter, hoping it is iterable
    # if not, a TypeError occurs
    try:
        maxWord = ''
        empty = True       # is the sequence empty?
        nonstring = False  # does the sequence contain a nonstring?

        for w in words:    # this might cause a TypeError
            empty = False  # if we got this far, there is at least one item
            if not isinstance(w, str):
                nonstring = True  # this is a problem
            else:
                if len(w) > len(maxWord):
                    maxWord = w

    except TypeError:   # problem with the for loop
        raise TypeError('sequence must be iterable')

    if empty:
        raise ValueError('sequence must be nonempty')

    if nonstring:
        raise TypeError('sequence elements must be strings')

    # if we get this far, we have a legitimate answer
    return maxWord