Programming Python, 4th Edition

Chapter 3. Script Execution Context

“I’d Like to Have an Argument, Please”

Python scripts don’t run in a vacuum (despite what you may have heard). Depending on platforms and startup procedures, Python programs may have all sorts of enclosing context—information automatically passed in to the program by the operating system when the program starts up. For instance, scripts have access to the following sorts of system-level inputs and interfaces:

Current working directory

os.getcwd gives access to the directory from which a script is started, and many file tools use its value implicitly.

Command-line arguments

sys.argv gives access to words typed on the command line that are used to start the program and that serve as script inputs.

Shell variables

os.environ provides an interface to names assigned in the enclosing shell (or a parent program) and passed in to the script.

Standard streams

sys.stdin, stdout, and stderr export the three input/output streams that are at the heart of command-line shell tools, and can be leveraged by scripts with print options, the os.popen call and subprocess module introduced in Chapter 2, the io.StringIO class, and more.

Such tools can serve as inputs to scripts, configuration parameters, and so on. In this chapter, we will explore all these four context’s tools—both their Python interfaces and their typical roles.

Current Working Directory

The notion of the current working directory (CWD) turns out to be a key concept in some scripts’ execution: it’s always the ...