1. What a stock screener is and why it saves you time

Level 4: Stock Screening

Filtering 500 companies by hand, checking one by one whether they meet a set of criteria, is simply not viable. A stock screener is exactly the tool that solves that problem: a filter that sweeps a universe of shares and returns only those that meet the conditions you define — a minimum market capitalization, a maximum price/earnings ratio, a sector, or any technical indicator.

There are ready-made free screeners, such as Finviz or TradingView, and they are a good starting point. But almost all of them cap real customization unless you pay for a subscription — and as soon as your filtering criterion combines several specific conditions (a particular moving average, a ratio computed your way, an indicator you defined yourself), the only way to have full control is to program it yourself.

The good news is that a home-made screener does not require much infrastructure: with the data you already know how to download (Level 3) and a few lines of pandas, you can build a working one in a Jupyter notebook. The rest of this level builds three real screeners, from the simplest to the most sophisticated, over the 500 companies of the S&P 500.

The first step, common to all three, is having clean data to start from — exactly the same download and cleaning process as in Level 3 (scraping tickers from Wikipedia, downloading with yfinance, dropna to remove gaps). From there, each screener adds its own filtering criterion on top of that same data.