What does "i" represent in Python .pyi extension?

I think the i in .pyi stands for "Interface"

Definition for Interface in Java:

An interface in the Java programming language is an abstract type that is used to specify a behaviour that classes must implement

• From Python typeshed github repository:

Each Python module is represented by a .pyi "stub". This is a normal Python file (i.e., it can be interpreted by Python 3), except all the methods are empty.

• In 'Mypy' repository, they explicitly mention "stub" files as public interfaces:

A stubs file only contains a description of the public interface of the module without any implementations.

Because "Interfaces" do not exist in Python (see this SO question between Abstract class and Interface) I think the designers intended to dedicate a special extension for it.

pyi implements "stub" file (definition from Martin Fowler)

Stubs: provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test.

But people are more familiar with Interfaces than "stub" files, therefore it was easier to choose .pyi rather than .pys to avoid unnecessary confusion.

Apparently PyCharm creates .pyi file for its own purposes:

The *.pyi files are used by PyCharm and other development tools to provide more information, such as PEP 484 type hints, than it is able to glean from introspection of extension types and methods. They are not intended to be imported, executed or used for any other purpose other than providing info to the tools. If you don't use use a tool that makes use of .pyi files then you can safely ignore this file.

See: https://www.python.org/dev/peps/pep-0484/ https://www.jetbrains.com/help/pycharm/2016.1/type-hinting-in-pycharm.html

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