How to Calculate Percentile Latency in Python

import random
import statistics

def generate_latency_samples(n=1000):
    """Generate realistic mock latency data (ms) with occasional spikes."""
    samples = []
    for _ in range(n):
        # Normal case: ~50ms with jitter
        base = random.gauss(50, 5)
        # 2% spike chance: slow downstream or GC pause
        if random.random() < 0.02:
            base += random.uniform(100, 500)
        samples.append(max(0, base))
    return samples

def percentile(data, p):
    """Compute p-th percentile of a list."""
    sorted_data = sorted(data)
    k = (len(sorted_data) - 1) * (p / 100)
    f = int(k)
    c = f + 1
    if c >= len(sorted_data):
        return sorted_data[f]
    return sorted_data[f] + (k - f) * (sorted_data[c] - sorted_data[f])

if __name__ == "__main__":
    random.seed(42)
    latencies = generate_latency_samples()
    for p in (50, 90, 95, 99):
        print(f"p{p}: {percentile(latencies, p):.2f} ms")
    print(f"mean: {statistics.mean(latencies):.2f} ms")