I am trying to understand the lower level implementations of python 3. There is one module named _posixsubprocess used by the subprocess module. I tried to find the location of this module in my system and found that it's a stub file.
Could someone guide me as I have no idea about what are the stub files and how are they implemented at the lower level?
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.
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:
# type: ... comment syntax