How to use the jellyfish.porter_stem function in jellyfish

def keywordize(str):
    """
    Splits a string into words, removes common stopwords, stems and removes
    duplicates.
    """
    import jellyfish
    return set([jellyfish.porter_stem(word.lower().encode('ascii',
                                                          'ignore'))
                for word in tokenize(str)
                if (word.isalpha() or word.isdigit()) and
                word.lower() not in stop_words])