Building a Real-Time Price Anomaly Detector with Python, SerpApi, and Robust Statistics

Modern price monitoring systems need to do more than tell you that a price changed. A single abnormal listing, a scraped error, or a temporary outlier can make a traditional threshold-based detector fire an alert when nothing meaningful happened. In this project, I built a lightweight real-time price anomaly detector in Python that combines: A rolling median baseline Median Absolute Deviation (MAD) Robust Z-scores Short-term percentage returns Trend confirmation Alert cooldowns The goal is simple: detect meaningful price movements without overreacting to noisy observations. Note: This project monitors retail prices from Google Shopping results through SerpApi. It is a retail-price monitoring example, not a financial exchange-data feed. What we're building The pipeline looks like this: ┌──────────────────────┐ │ SerpApi / Shopping │ └──────────┬───────────┘ │ ▼ ┌──────────────────┐ │ Price Extraction │ │ + Validation │ └────────┬─────────┘ │ ▼ ┌────────────────────┐ │ Rolling Price │ │ History │ └────────┬───────────┘ │ ┌───────────┼───────────┐ ▼ ▼ ▼ Median MAD Return % │ │ │ └───────────┼───────────┘ ▼ Robust Z-score │ ▼ Trend Confirmation │ ▼ Signal Engine │ ▼ Alert Cooldown The implementation is intentionally small and interpretable. The complete engine is built around a single PriceAlertEngine class and a compact AnomalyResult data structure. Why not just use standard deviation? A common first implementation is: price > mean + 3 * standard_deviation The problem is that standard deviation is sensitive to extreme observations. Suppose your historical prices are: 990, 995, 999, 1001, 1005 Then one bad observation such as: 1500 can distort the mean and standard deviation. That can move your detection boundary away from the actual market behavior you are trying to model. For a noisy retail environment, a more robust baseline is useful. That's where median and Median Absolute Deviation come in. 1. Building a rolling median baseline Instead of storing an unlimited stre