python/mypy

Mypy is a static type checker for Python, a tool that reads your Python code and checks whether you are using variables and functions correctly, before you ever run the program. Python normally only reveals type-related mistakes at runtime (for example, trying to add a number to a piece of text), which can mean bugs surface only in production. Mypy catches those errors at development time, acting like a spell-checker for your code's logic rather than its spelling. The way it works is through "type hints," optional labels you add to your Python code that describe what kind of data a variable or function expects. Once those hints are in place, you run the mypy command on your program and it reports any mismatches. Importantly, adding type hints does not change how Python runs your code, you can still execute it normally with the regular Python interpreter. Mypy also supports "gradual typing," meaning you can introduce hints to an existing project file by file, at your own pace, rather than converting everything at once. For teams managing large codebases, mypy offers a daemon mode that keeps a background process watching for changes, delivering near-instant feedback instead of re-scanning the entire project on every check. It integrates with common editors including VS Code, PyCharm, Vim, and Emacs, so errors can appear directly in your editor as you write. Mypy itself is written in Python and is compiled using a related tool called Mypyc, which makes it roughly four times faster than running it interpreted. You would reach for mypy when you want to make a Python project more reliable, easier to refactor, and clearer for collaborators to understand, especially as the codebase grows beyond what a single person can hold in their head.