Reading builtin python modules

I dont understand how to read the code in the builtin python modules. I know how to find out whats in a module for example,

import os;

dir(os)
But when I try to look for example for the function listdir I cannot find a def listdir to read what it actually does.

The source for os.listdir is almost certainly written in c. Python would load it from a shared object or dynamic link library. If you really want it, download the python source and hunt for it.

...Trying my advice...
I think it starts with this line in file posixmodule.c.

posix_listdir(PyObject *self, PyObject *args, PyObject *kwargs)

Ok thank you for the response but I still dont quite follow. OS.py is written in python right? so when i import os im importing os.py, why then can i not find the functions listed inside os.py that i can see when i do a dir(os)

An os.py on my system:
/usr/lib/python2.7/os.py


Read this manual.

That goes a little over my head at this stage in my learning python. Are you basically saying that the functions arent in os.py but os.py loads some c code that includes the os.blar functions?

Yes.