Python application cross-platform single instance.

single_instance.py

import sys
import os

try:
    import fcntl
except ImportError:
    fcntl = None

LOCK_PATH = os.path.join(os.path.abspath(os.path.dirname(sys.argv[0])), "lock")
OS_WIN = False
if 'win32' in sys.platform.lower():
    OS_WIN = True


class SingleInstance:
    def __init__(self):
        self.fh = None
        self.is_running = False
        self.do_magic()

    def do_magic(self):
        if OS_WIN:
            try:
                if os.path.exists(LOCK_PATH):
                    os.unlink(LOCK_PATH)
                self.fh = os.open(LOCK_PATH, os.O_CREAT | os.O_EXCL | os.O_RDWR)
            except EnvironmentError as err:
                if err.errno == 13:
                    self.is_running = True
                else:
                    raise
        else:
            try:
                self.fh = open(LOCK_PATH, 'w')
                fcntl.lockf(self.fh, fcntl.LOCK_EX | fcntl.LOCK_NB)
            except EnvironmentError as err:
                if self.fh:
                    self.is_running = True
                else:
                    raise

    def clean_up(self):
        # this is not really needed
        try:
            if OS_WIN and self.fh:
                self.fh.close()
                os.unlink(LOCK_PATH)
            elif self.fh:
                fcntl.lockf(self.fh, fcntl.LOCK_UN)
                self.fh.close() # ???
                os.unlink(LOCK_PATH)
        except Exception as err:
            # logger.exception(err)
            pass


if __name__ == "__main__":
    import time

    si = SingleInstance()
    try:
        if si.is_running:
            sys.exit("This app is already running!")
        time.sleep(20)
        # do other stuff
    finally:
        si.clean_up()

Tested on Python 2.7. The sleep is not needed, I included it for further testing.