Core Python Programming (2nd Edition)

Calling sys.exit() causes the Python interpreter to quit. Any integer argument to exit() will
be returned to the caller as the exit status, which has a default value of 0.
1 #!/usr/bin/env python
2
3 import sys
4
5 def usage():
6 print 'At least 2 arguments (incl. cmd name).'
7 print 'usage: args.py arg1 arg2 [arg3... ]'
8 sys.exit(1)
9
10 argc = len(sys.argv)
11 if argc < 3:
12 usage()
13 print "number of args entered:", argc
14 print "args (incl. cmd name) were:", sys.argv
Many command-line-driven programs test the validity of the input before proceeding with the core
functionality of the script. If the validation fails at any point, a call is made to a usage() function to
inform the user what problem caused the error as well as a usage "hint" to aid the user so that he or
she will invoke the script properly the next time.
14.7.2. sys.exitfunc()
sys.exitfunc() is disabled by default, but can be overridden to provide additional functionality, which
takes place when sys.exit() is called and before the interpreter exits. This function will not be passed
any arguments, so you should create your function to take no arguments.
If sys.exitfunc has already been overridden by a previously defined exit function, it is good practice to
also execute that code as part of your exit function. Generally, exit functions are used to perform some
type of shutdown activity, such as closing a file or network connection, and it is always a good idea to
complete these maintenance tasks, such as releasing previously held system resources.
Here is an example of how to set up an exit function, being sure to execute one if one has already been
set:
import sys
prev_exit_func = getattr(sys, 'exitfunc', None)
def my_exit_func(old_exit = prev_exit_func):
# :
# perform cleanup
# :
if old_exit is not None and callable(old_exit):
old_exit()
sys.exitfunc = my_exit_func