Doing something before program exit

How can you have a function or something that will be executed before your program quits? I have a script that will be constantly running in the background, and I need it to save some data to a file before it exits. Is there a standard way of doing this?

Check out the atexit module:

http://docs.python.org/library/atexit.html

For example, if I wanted to print a message when my application was terminating:

import atexit

def exit_handler():
    print 'My application is ending!'

atexit.register(exit_handler)

Just be aware that this works great for normal termination of the script, but it won't get called in all cases (e.g. fatal internal errors).

If you want something to always run, even on errors, use try: finally: like this -

def main():
    try:
        execute_app()
    finally:
        handle_cleanup()

if __name__=='__main__':
    main()

If you want to also handle exceptions you can insert an except: before the finally:

If you stop the script by raising a KeyboardInterrupt (e.g. by pressing Ctrl-C), you can catch that just as a standard exception. You can also catch SystemExit in the same way.

try:
    ...
except KeyboardInterrupt:
    # clean up
    raise

I mention this just so that you know about it; the 'right' way to do this is the atexit module mentioned above.

If you have class objects, that exists during the whole lifetime of the program, you can also execute commands from the classes with the __del__(self) method:

class x:
def __init__(self):
    while True:
        print ("running")
        sleep(1)

def __del__(self):
    print("destructuring")


a = x()

this works on normal program end as well if the execution is aborted, for sure there will be some exceptions:

running
running
running
running
running
Traceback (most recent call last):
  File "x.py", line 14, in <module>
    a = x()
  File "x.py", line 8, in __init__
    sleep(1)
KeyboardInterrupt
destructuring