Treating a script as its own mutex lock.

locking.py

import contextlib
import fcntl
import time


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


@contextlib.contextmanager
def mutex(filename):
    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():
    with mutex(__file__):
        time.sleep(5)


if __name__ == '__main__':
    main()