code: def function(self, function_name, word):
          
          word = self._parse_filter_word(word)
  
          self._add_filter(
              *self._prepare_function(function_name, self._attribute, word,
                                      self._negation))
          return self
docs: Apply a function on given word
  
          :param str function_name: function to apply
          :param str word: word to apply function on
          :rtype: Query