Programming Python, 3rd Edition

Introducing the os Module

As mentioned, os is the larger of the two core system modules. It contains all of the usual operating-system calls you may have used in your C programs and shell scripts. Its calls deal with directories, processes, shell variables, and the like. Technically, this module provides POSIX tools—a portable standard for operating-system calls—along with platform-independent directory processing tools as the nested module os.path. Operationally, os serves as a largely portable interface to your computer’s system calls: scripts written with os and os.path can usually be run unchanged on any platform.

In fact, if you read the os module’s source code, you’ll notice that it really just imports whatever platform-specific system module you have on your computer (e.g., nt, mac, posix). See the os.py file in the Python source library directory—it simply runs a from* statement to copy all names out of a platform-specific module. By always importing os rather than platform-specific modules, though, your scripts are mostly immune to platform implementation differences. On some platforms, os includes extra tools available just for that platform (e.g., low-level process calls on Unix); by and large, though, it is as cross-platform as it is technically feasible.

The Big os Lists

Let’s take a quick look at the basic interfaces in os. As a preview, Table 3-1 summarizes some of the most commonly used tools in the os module organized by functional area.

Table 3-1. Commonly used os ...