Return all words in dictionary file as set.

def read_dictionary_file(dictionary_path):
    """Return all words in dictionary file as set."""
    try:
        return _user_dictionary_cache[dictionary_path]
    except KeyError:
        if dictionary_path and os.path.exists(dictionary_path):
            with open(dictionary_path, "rt") as dict_f:
                words = set(re.findall(r"(\w[\w']*\w|\w)",
                                       " ".join(dict_f.read().splitlines())))
                return words

        return set()