How to implement a token bucket rate limiter in Python

import time
import threading

class TokenBucketRateLimiter:
    def __init__(self, capacity, refill_rate):
        self.capacity = capacity
        self.refill_rate = refill_rate
        self.tokens = capacity
        self.last_refill_time = time.time()
        self.lock = threading.Lock()

    def allow_request(self, key):
        with self.lock:
            now = time.time()
            elapsed = now - self.last_refill_time
            self.tokens = min(self.capacity, self.tokens + elapsed * self.refill_rate)
            self.last_refill_time = now

            if self.tokens >= 1:
                self.tokens -= 1
                return True
            return False

if __name__ == "__main__":
    limiter = TokenBucketRateLimiter(capacity=3, refill_rate=1)
    keys = ["user:123", "user:123", "user:123", "user:123"]
    
    for key in keys:
        allowed = limiter.allow_request(key)
        print(f"{key}: {'Allowed' if allowed else 'Rate limited'} (tokens left: {limiter.tokens:.2f})")
    
    time.sleep(1)
    allowed = limiter.allow_request("user:123")
    print(f"After refill - user:123: {'Allowed' if allowed else 'Rate limited'} (tokens left: {limiter.tokens:.2f})")