def setdefault(self, key, default=None):
if key in self:<EOL><INDENT>return self[key]<EOL><DEDENT>self._setitem(key, default)<EOL>self._list_add(key)<EOL>return default<EOL>
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``.