Question

I am using file locking in python script (to control single instance of it execution).
http://code.google.com/p/pylockfile/
I release lock in finally code block.
But if script closed, for example, closing the terminal running it, the finally block doesn’t execute and the file stays locked.

How to catch python script destructor event in any case?

Answer 1

See this blog post regarding this subject. It uses the win32api when under Windows, while under Linux the SIGTERM signal is caught. To verify its working, it might be helpful to write something to a file in the on_exit handler like as done below. As the snippet is quite brief, I’ll just include it (full props to the blog author):

import os, sys
def set_exit_handler(func):
    if os.name == "nt":
        try:
            import win32api
            win32api.SetConsoleCtrlHandler(func, True)
        except ImportError:
            version = '.'.join(map(str, sys.version_info[:2]))
            raise Exception('pywin32 not installed for Python ' + version)
    else:
        import signal
        signal.signal(signal.SIGTERM, func)

if __name__ == '__main__':
    def on_exit(sig, func=None):
        f = open('log.txt', 'w')
        f.write('shutdown...')
        f.close()
        sys.exit()
    set_exit_handler(on_exit)
    print 'Press  to quit'
    raw_input()
    print 'quit!'

If you will close the terminal running that program, it will create a file to verify the callback functionality.