What is the use of stub files (.pyi ) in python?

_posixsubprocess

The file you are referencing is a Python module written in C. It's not a "stub" file. The real implementation can be found in the stdlib at Modules/_posixsubprocess.c. You can see how writing a C/C++ extension is written by having a look at Building C and C++ Extensions. This should help you understanding the code in _posixsubprocess.c.

In order to add type-hints to that file (which is an "Extension Module" as it is written in C), the type hints are added to a "stub" file with the extension .pyi.

That file can be found in the typeshed which is a collection of stub files. The typeshed also contains stubs for third-party modules which is a historical remnant. That is no longer needed since PEP-561 has been adopted.

Concerning stub/pyi files

Stub files contain type-hinting information of normal Python modules. The full official documentation can be found in the section about stub-files in PEP-484.

For example, if you have a Python module mymodule.py like this:

def myfunction(name):   return "Hello " + name

Then you can add type-hints via a stub-file mymodule.pyi. Note that here the ellipsis (...) is part of the syntax, so the code-block below really shows the complete file contents:

def myfunction(name: str) -> str: ...

They look very similar to C header files in that they contain only the function signatures, but their use is purely optional.

You can also add type hints directly in the .py module like the following:

def myfunction(name: str) -> str:   return "Hello " + name

But there are some cases where you want to keep them separate in stubs:

• You want to keep your code Python 2 compatible and don't like the # type: ... comment syntax
• You use function annotations for something else but still want to use type-hints
• You are adding type-hints into an existing code-base and want to keep code-churn in existing files minimal