code: def setdefault(self, key, default=None):
          
          if key in self:
              return self[key]
          else:
              self._setitem(key, default)
              self._list_add(key)
              return default
docs: If *key* is in the dictionary, return its value.  If not, insert *key*
          with a value of *default* and return *default*.  *default* defaults to
          ``None``.