Python Only Allow Single Instance to Run (or Kill Previous Instance)

Dec 13, 2020

Install pip install psutil

Store pid and command line of current running instance.

NOTE: This will not work in Windows due to fcntl. Refer to tendo.

import fcntlimport psutillockfile_fp = Nonedef check_instance_running(lockfile):    global lockfile_fp    lockfile_fp = open(lockfile, 'a')    try:        fcntl.lockf(lockfile_fp, fcntl.LOCK_EX | fcntl.LOCK_NB)        lockfile_fp.truncate()        # lockfile_fp.write(str(os.getpid()))        lockfile_fp.write(json.dumps({            'pid': os.getpid(),            'cmdline': psutil.Process(os.getpid()).cmdline()        }))        lockfile_fp.flush()        running = False    except IOError:        with open(lockfile, 'r') as fp:            # running =            running = json.loads(        # running = True    return running

Only allow single instance to run

lockfile = 'test.lock'data = check_instance_running(lockfile)if data:    raise Exception(f"Already running: {data['pid']}")print('START')while True:    print('.', end='', flush=True)    time.sleep(5)

Kill previous instance

lockfile = 'test.lock'data = check_instance_running(lockfile)if data:    p = psutil.Process(data['pid'])    # check cmdline, as pid could be recyled    cmdline = p.cmdline()    # check script and arguments must be the same    # if (p.cmdline() == data['cmdline']):        # check if same script file only: cmdline[0]==python, cmdline[1]==scriptname, cmdline[2..]==arguments    if len(cmdline) >= 2 and len(data['cmdline']) >= 2 and cmdline[1] == data['cmdline'][1]:        print(f"Kill Previous Process: {data['pid']}")        p.terminate()        p.wait()        # create new lock file        util.check_instance_running(lockfile)print('START')while True:    print('.', end='', flush=True)    time.sleep(5)

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✨ By Desmond Lua

A dream boy who enjoys making apps, travelling and making youtube videos. Follow me on @d_luaz

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