Programming Python, Second Edition

Program Exits

As we’ve seen, unlike C there is no “main” function in Python -- when we run a program, we simply execute all the code in the top-level file, from top to bottom (i.e., in the filename we listed in the command line, clicked in a file explorer, and so on). Scripts normally exit when Python falls off the end of the file, but we may also call for program exit explicitly with the built-in sys.exit function:

>>> sys.exit(  )           # else exits on end of script

Interestingly, this call really just raises the built-in SystemExit exception. Because of this, we can catch it as usual to intercept early exits and perform cleanup activities; if uncaught, the interpreter exits as usual. For instance:

C:\...\PP2E\System>python
>>> import sys
>>> try:
...     sys.exit(  )              # see also: os._exit, Tk(  ).quit(  )
... except SystemExit:
...     print 'ignoring exit'
...
ignoring exit
>>>

In fact, explicitly raising the built-in SystemExit exception with a Python raise statement is equivalent to calling sys.exit. More realistically, a try block would catch the exit exception raised elsewhere in a program; the script in Example 3-11 exits from within a processing function.

Example 3-11. PP2E\System\Exits\testexit_sys.py

def later(  ):
    import sys
    print 'Bye sys world'
    sys.exit(42)
    print 'Never reached'

if __name__ == '__main__': later(  )

Running this program as a script causes it to exit before the interpreter falls off the end of the file. But because sys.exit raises a Python exception, importers of its function ...