Python Type Hints That Actually Catch Bugs (Not Just Satisfy mypy)

Most type hint guides teach you syntax. This one is about which annotations actually prevent bugs. After two years of adding types to codebases that didn't have them — and watching which annotations caught real issues and which were just ceremony — I've settled on four patterns. They're not the four most commonly taught. They're the four that have caught production bugs before they shipped. The gap between "mypy passes" and "your code is correct" mypy can pass on code that will throw a TypeError