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.