Return a circuit breaker for the given ID.

def context(self, id):
        """Return a circuit breaker for the given ID."""
        if id not in self.circuits:
            self.circuits[id] = self.factory(self.clock, self.log.getChild(id),
                                             self.error_types, self.maxfail,
                                             self.reset_timeout,
                                             self.time_unit,
                                             backoff_cap=self.backoff_cap,
                                             with_jitter=self.with_jitter)
        return self.circuits[id]