Core Python Programming (2nd Edition)

14.7. Terminating Execution
Clean execution occurs when a program runs to completion, where all statements in the top level of
your module finish execution and your program exits. There may be cases where you may want to exit
from Python sooner, such as a fatal error of some sort. Another case is when conditions are not
sufficient to continue execution.
In Python, there are varying ways to respond to errors. One is via exceptions and exception handling.
Another way is to construct a "cleaner" approach so that the main portions of code are cordoned off with
if statements to execute only in non-error situations, thus letting error scenarios terminate "normally."
However, you may also desire to exit the calling program with an error code to indicate that such an
event has occurred.
14.7.1. sys.exit() and SystemExit
The primary way to exit a program immediately and return to the calling program is the exit() function
found in the sys module. The syntax for sys.exit() is:
sys.exit(status=0)
When sys.exit() is called, a SystemExit exception is raised. Unless monitored (in a try statement with
an appropriate except clause), this exception is generally not caught or handled, and the interpreter
exits with the given status argument, which defaults to zero if not provided. System Exit is the only
exception that is not viewed as an error. It simply indicates the desire to exit Python.
One popular place to use sys.exit() is after an error is discovered in the way a command was invoked,
in particular, if the arguments are incorrect, invalid, or if there are an incorrect number of them. The
following Example 14.4 (args.py) is just a test script we created to require that a certain number of
arguments be given to the program before it can execute properly.
Executing this script we get the following output:
$ args.py
At least 2 arguments required (incl. cmd name).
usage: args.py arg1 arg2 [arg3... ]
$ args.py XXX
At least 2 arguments required (incl. cmd name).
usage: args.py arg1 arg2 [arg3... ]
$ args.py 123 abc
number of args entered: 3
args (incl. cmd name) were: ['args.py', '123', 'abc']
$ args.py -x -2 foo
number of args entered: 4
args (incl. cmd name) were: ['args.py', '-x', '-2',
'foo']
Example 14.4. Exiting Immediately (args.py)