Python Cookbook

Determining if Another Instance of a Script Is Already Running in Windows

Credit: Bill Bell

Problem

In a Win32 environment, you want to ensure that only one instance of a script is running at any given time.

Solution

Many tricks can be used to avoid starting multiple copies of an application, but they’re all quite fragile—except those based on a mutual-exclusion (mutex) kernel object, such as this one. Mark Hammond’s precious win32all package supplies all the needed hooks into the Windows APIs to let us exploit a mutex for this purpose:

from win32event import CreateMutex
from win32api import GetLastError
from winerror import ERROR_ALREADY_EXISTS
from sys import exit

handle = CreateMutex(None, 1, 'A unique mutex name')

if GetLastError(  ) == ERROR_ALREADY_EXISTS:
    # Take appropriate action, as this is the second
    # instance of this script; for example:
    print 'Oh! dear, I exist already.'
    exit(1)
else:
    # This is the only instance of the script; let
    # it do its normal work. For example:
    from time import sleep
    for i in range(10):
        print "I'm running",i
        sleep(1)
    print "I'm done"

Discussion

The string 'A unique mutex name' must be chosen to be unique to this script, but it should not be dynamically generated, as it must be the same for all potential simultaneous instances of the same script. A fresh, globally unique ID generated at script-authoring time would be a good choice. According to the Windows documentation, the string can contain any characters except backslashes (\). On Windows platforms ...