What is a PYI file? A PYI file is a code stub used to specify type hints for a Python module. Python type checkers can read PYI files and use them to warn developers of potential type mismatches. PYI files have the same syntax as regular Python modules; they are typically stored alongside the module for which they contain type hints.

More Information PYI file open in Microsoft Visual Studio Code 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.