Additional object or objects required by this object.

def requires(self):
        """Additional object or objects required by this object."""
        # NOTE: spec says this can also be a list of strings
        value = self._schema.get("requires", {})
        if not isinstance(value, (basestring, dict)):
            raise SchemaError(
                "requires value {0!r} is neither a string nor an"
                " object".format(value))
        return value