def key(self, direction, mechanism, purviews=False, _prefix=None):
        """Cache key. This is the call signature of |Subsystem.find_mice()|."""
        return "subsys:{}:{}:{}:{}:{}".format(
            self.subsystem_hash, _prefix, direction, mechanism, purviews)
Cache key. This is the call signature of |Subsystem.find_mice()|.