old_contents: class WordHelper(object):
    def __init__(self, format_helper):
      self.format_helper = format_helper
      self.word_category_words = {}
  
    def add_word(self, word_category, word):
      self.word_category_words.setdefault(word_category, set([]))
      self.word_category_words[word_category].add(word)
  
    def get_template_element_words(self, template_element):
      template_element_word_categories = self.format_helper.decompose_template_element(template_element)
      words = self.word_category_words[template_element_word_categories[0]]
      for tewc in template_element_word_categories[1:]:
        words = words.intersection(self.word_category_words[tewc])
      return words
  
    @property
    def word_categories(self):
      return self.word_category_words.keys()
  
new_contents: 
  
  class WordHelper(object):
  
    def __init__(self, format_helper):
      self.format_helper = format_helper
      self.word_category_words = {}
  
    def add_word(self, word_category, word):
      self.word_category_words.setdefault(word_category, set([]))
      self.word_category_words[word_category].add(word)
  
    def get_template_element_words(self, template_element):
      template_element_word_categories = (
          self.format_helper.decompose_template_element(template_element))
      words = self.word_category_words[template_element_word_categories[0]]
      for tewc in template_element_word_categories[1:]:
        words = words.intersection(self.word_category_words[tewc])
      return words
  
    @property
    def word_categories(self):
      return self.word_category_words.keys()
  
subject: Update word_helper.py