Python: single instance locking for your scripts

instance_lock.py

"""Single instance locking for your scripts.

Usage:
    from instance_lock import lock_instance

    def i_am_busy():
        print "I'm busy!"

    with lock_instance(locked_callback=i_am_busy):
        do_something_long()
"""

import os
import fcntl
import errno
from contextlib import contextmanager


@contextmanager
def lock_instance(filename, locked_callback=None):
    """Gets an instance lock.

    Arguments:
        filename: a string with lock filename;
        locked_callback: a function to be called if lock fails.
    """
    assert locked_callback is None or callable(locked_callback)
    lock_fp, need_remove = None, False
    try:
        lock_fp = open(filename, 'w')
        fcntl.flock(lock_fp, fcntl.LOCK_NB | fcntl.LOCK_EX)
        need_remove = True
        yield lock_fp
    except IOError as ex_info:
        if locked_callback is not None and ex_info.errno == errno.EAGAIN:
            locked_callback()
        else:
            raise
    finally:
        if lock_fp:
            lock_fp.close()
        if need_remove:
            os.remove(filename)