def _get_instance_key(self, host, namespace, wmi_class, other=None):
        """
        Return an index key for a given instance. Useful for caching.
        """
        if other:
            return "{host}:{namespace}:{wmi_class}-{other}".format(
                host=host, namespace=namespace, wmi_class=wmi_class, other=other
            )

        return "{host}:{namespace}:{wmi_class}".format(host=host, namespace=namespace, wmi_class=wmi_class)