.pyi

Extension: Python Interface Definition

Developer: Python Software Foundation

A .pyi file, short for Python Interface Definition, is a special type of file used primarily for static type checking in Python. Unlike regular .py files which contain executable code, .pyi files only provide type hints. These hints describe the expected types of variables, function arguments, and return values within a module. Think of them as blueprints for the structure and data types within a library or module, allowing tools like MyPy to verify that your code interacts correctly with those libraries without needing to understand the library’s internal implementation. This helps catch potential type errors before runtime, leading to more robust and reliable code. They are especially useful when working with external libraries where you might not have access to the source code.

You cannot directly run or execute a .pyi file; they are designed solely for static analysis by type checkers. To open a .pyi file, you can use any plain text editor, such as Notepad (Windows), TextEdit (macOS), or VS Code, Sublime Text, Atom, or similar code editors. These editors will display the type annotations within the file, allowing you to inspect the expected types and interfaces. Note that while you *can* modify a .pyi file, it’s generally not recommended unless you are creating or maintaining type stubs for a library. Incorrectly modifying a .pyi file can lead to inaccurate type checking results. Generally, type checkers will prioritize a .pyi file over a corresponding .py file if both exist, making the .pyi the authoritative source for type information.