.PYI File Extension

Because Python is a dynamically-typed programming language, the Python interpreter performs type checking only when code runs. However, some Python code editors allow developers to manually define and check a module's types while coding, using PYI files. That way, developers can catch and fix type errors before attempting to run their program.

For example, JetBrains PyCharm allows developers to check types using PYI files. If a developer's code creates a mismatch between a module's actual and defined types, PyCharm will warn the developer.

To open a .pyi file, you need compatible software that supports this file format.