How to implement a circuit breaker in Python

class CircuitBreaker:
    def __init__(self, failure_threshold=3, timeout=5):
        self.failure_threshold = failure_threshold
        self.timeout = timeout
        self.failure_count = 0
        self.last_failure_time = None
        self.state = "CLOSED"

    def call(self, mock_downstream):
        if self.state == "OPEN":
            if self._timeout_elapsed():
                self.state = "HALF_OPEN"
            else:
                return "Service unavailable (circuit open)"

        try:
            result = mock_downstream()
            if self.state == "HALF_OPEN":
                self.failure_count = 0
                self.state = "CLOSED"
            return result
        except Exception:
            self.failure_count += 1
            self.last_failure_time = __import__("time").time()
            if self.state == "HALF_OPEN" or self.failure_count >= self.failure_threshold:
                self.state = "OPEN"
            return "Call failed"

    def _timeout_elapsed(self):
        import time
        return (time.time() - self.last_failure_time) > self.timeout


def mock_downstream_v1():
    raise Exception("Downstream error")

def mock_downstream_v2():
    return "Success response"

if __name__ == "__main__":
    circuit = CircuitBreaker(failure_threshold=2, timeout=1)
    print(circuit.call(mock_downstream_v1))  # Call failed
    print(circuit.call(mock_downstream_v1))  # Call failed, opens circuit
    print(circuit.call(mock_downstream_v2))  # Service unavailable (circuit open)