python - Doing something before program exit

Question

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?

Answer 1

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).

Answer 2

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:

Answer 3

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.

Answer 4

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