Make a python script single-instance

Reverend Jim 1 2K Views

Sometimes you want to ensure that only one copy of a script can be run at time. Windows provides a facility called a mutex and you can use it to prevent multiple instances.

To create the mutex, all I have to do is

varname = GetMutex()

If the mutex already exists then IsRunning() will return True so the line

if (myapp := GetMutex()).IsRunning():

both creates the mutex (maybe) and tests if the app is already running. Note this this uses the walrus (:=) works only in Python 3.8.2 or newer.

"""                                                                             
    Name:                                                                       

        GetMutex.py                                                             

    Description:                                                                

        Provides a method by which an application can ensure that only one      
        instance of it can be running at any given time.                        

    Usage:                                                                      

        if (app := GetMutex()).IsRunning():                                     
            print("Application is already running")                             
            sys.exit()                                                          
                                                                             """

import os
import sys

from win32event import CreateMutex
from win32api   import CloseHandle, GetLastError
from winerror   import ERROR_ALREADY_EXISTS

class GetMutex:
    """ Limits application to single instance """

    def __init__(self):
        thisfile   = os.path.split(sys.argv[0])[-1]
        self.name  = thisfile + "_{D0E858DF-985E-4907-B7FB-8D732C3FC3B9}"
        self.mutex = CreateMutex(None, False, self.name)
        self.error = GetLastError()

    def IsRunning(self):
        return (self.error == ERROR_ALREADY_EXISTS)

    def __del__(self):
        if self.mutex: CloseHandle(self.mutex)

if __name__ == "__main__":

    # check if another instance of this program is running
    if (myapp := GetMutex()).IsRunning():
        print("Another instance of this program is already running")
        sys.exit(0)

    # not running, safe to continue...
    print("No another instance is running, can continue here")

    try:
        while True: pass
    except KeyboardInterrupt: pass
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