Detect if another copy of a script is running from within the script

[gw1500se]

I have a situation where an application can launch a python script at any time. It can also launch the script multiple times almost simultaneously. I tried the following code in the script:

#prevent multiple instances
locker=tmpdir+"/spamcop.lock"
if (os.path.exists(locker)):
    exit
lockfile=open(locker,"w");
lockfile.write("lock")
lockfile.close()

This does not seem to work. I suspect that the multiple scripts are launched so close that both fail the 'if' statement. Is there a better way, within the script itself, to prevent multiple copies running? TIA.

[Skaperen]

do you mean multiple processes on the same computer?

what if i have a copy of your script on my laptop (computer) and run your script at the same time? does this matter?

[Gribouillis]

You could perhaps use a library such as filelock, something like (untested code)

import filelock
import sys

def main():
    ...

if __name__ == '__main__':
    lock = filelock.FileLock('mylock.lock')
    try:
        with lock.acquire(blocking=False):
            main()
    except filelock.Timeout:
        sys.exit(1)

[deanhystad]

What is tempdir? If it is a temporary directory created using the tempfile module each instance of the script has its own unique directory. Post more of your code to provide context.

[Skaperen]

are you assuming your particular OS will refuse to allow a directory to be created when the request names an existing directory? FYI, i once (back when i was in college) used (by remote dial up) an OS on a fairly large computer (considered a mainframe back then) where that is not true.