code: def _normalize(self, key, value):
          
  
          # None value should not be converted by normalizer
          if value is None:
              return None
  
          normalize_func = getattr(self, 'normalize_{0}'.format(key),
                                   lambda x: x)
  
          return normalize_func(value)
docs: Use normalize_<key> methods to normalize user input. Any user
          input will be normalized at the moment it is used as filter,
          or entered as a value of Task attribute.