How to mock Prometheus alert rule thresholds in Python

import time
import random


class MetricsStore:
    def __init__(self):
        self.metrics = {}

    def set_metric(self, name, value, labels=None):
        key = (name, tuple(sorted((labels or {}).items())))
        self.metrics[key] = value

    def get_metric(self, name, labels=None):
        key = (name, tuple(sorted((labels or {}).items())))
        return self.metrics.get(key, 0.0)


class PrometheusAlertRule:
    def __init__(self, expr_name, threshold, duration_seconds, metrics_store):
        self.metric_name = expr_name
        self.threshold = threshold
        self.duration = duration_seconds
        self.metrics = metrics_store
        self.firing_since = None

    def evaluate(self, current_time=None):
        current_time = current_time or time.time()
        value = self.metrics.get_metric(self.metric_name)

        if value > self.threshold:
            if self.firing_since is None:
                self.firing_since = current_time
            sustained = current_time - self.firing_since
            if sustained >= self.duration:
                return f"FIRING: {self.metric_name}={value} exceeded {self.threshold} for {sustained:.1f}s"
        else:
            self.firing_since = None
        return f"OK: {self.metric_name}={value}"


if __name__ == "__main__":
    store = MetricsStore()
    rule = PrometheusAlertRule(expr_name="http_requests_total", threshold=100, duration_seconds=5, metrics_store=store)

    start = time.time()
    for tick in range(8):
        # Simulate increasing traffic
        store.set_metric("http_requests_total", 50 + tick * 30)
        print(f"t={tick}: {rule.evaluate(current_time=start + tick)}")
        time.sleep(0.5)