mypy

Mypy is a tool that checks your Python code for type errors before you run it. Instead of discovering bugs only when you execute your program, mypy reads your code and warns you about mistakes — like trying to add a number to text, or calling a method that doesn't exist on a particular object. This catches problems early, saving you debugging time.

Here's how it works in practice: you add optional type hints to your Python functions and variables, telling mypy what kinds of data they should contain. For example, you might write a function that says "this parameter should be an integer." Then you run mypy as a separate command-line tool, and it analyzes your entire codebase without actually executing it. If mypy finds a place where you're using a variable in a way that contradicts its type hint, it will flag that as an error. The good news is that these type hints are completely optional — your Python code runs exactly the same whether you add them or not.

Mypy is designed to fit naturally into how Python developers already work. You don't have to type-hint your entire project at once; you can add hints gradually, one function or file at a time. This makes it practical for large existing codebases or teams just getting started with type checking. The tool integrates with popular code editors like VS Code, PyCharm, and Vim, so you get real-time feedback as you type.

People use mypy because typed code is easier to understand, maintain, and refactor. When a colleague (or future you) reads a function signature with type hints, they immediately know what kind of data goes in and what comes out — no guessing required. Mypy also catches whole categories of bugs that might otherwise only appear in production, like calling methods on objects that don't have them, or passing the wrong number of arguments to a function.