code: def freeze(obj):
      
      if isinstance(obj, collections.Mapping):
          return FrozenDict({freeze(k): freeze(v) for k, v in six.iteritems(obj)})
      elif isinstance(obj, list):
          return FrozenList([freeze(e) for e in obj])
      else:
          return obj
docs: Transform tree of dict and list in read-only
      data structure.
      dict instances are transformed to FrozenDict, lists
      in FrozenList.