Programming Python, 3rd Edition

Script Execution Context

Python scripts don’t run in a vacuum. 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.

Such tools can serve as inputs to scripts, configuration parameters, and so on. In the next few sections, we will explore these context tools—both their Python interfaces and their typical roles.