Using a python script as its own mutex to prevent it executing concurrently with multiple instances of itself

selfmutex.py

#!/usr/bin/env python3

import contextlib
import fcntl
import sys
import time


class MutexException(Exception):
    """
    Failed to lock a file.
    """


@contextlib.contextmanager
def mutex(filename):
    """
    Using file identified by filename as mutex

    :param filename: The file we are going to use as mutex
    """
    with open(filename, 'r') as fh:
        try:
            try:
                fcntl.flock(fh, fcntl.LOCK_EX | fcntl.LOCK_NB)
            except IOError:
                raise MutexException("Failed to lock {}".format(filename))
            yield
        finally:
            fcntl.flock(fh, fcntl.LOCK_UN)


def main():
    print("Started! Doing work...")
    time.sleep(5)
    print("All done!")
    return 0


if __name__ == "__main__":
    try:
        with mutex(__file__):
            sys.exit(main())
    except MutexException:
        print("{} is already running, exiting".format(__file__),
              file=sys.stderr)
        sys.exit(2)