code: def _add_string_to_commastring(self, field, string):
          # type: (str, str) -> bool
          
          if string in self._get_stringlist_from_commastring(field):
              return False
          strings = '%s,%s' % (self.data.get(field, ''), string)
          if strings[0] == ',':
              strings = strings[1:]
          self.data[field] = strings
          return True
docs: Add a string to a comma separated list of strings
  
          Args:
              field (str): Field containing comma separated list
              string (str): String to add
  
          Returns:
              bool: True if string added or False if string already present