def register_type(self, key, *types):
        """
        Register special types for item in config. This is used for mixed type declaration.
        Note that is the type is declared as None, then we will not check type for this item.
        """
        assert key in self._config
        self._types[key] = set(types)

        Register special types for item in config. This is used for mixed type declaration.
        Note that is the type is declared as None, then we will not check type for this item.
        
metadrive/utils/config.py