lock_file

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Class LockError
Class LockFile
Examples

Lock file is a traditional means of synchronization among processes. In this module it is implemented as an empty regular file exclusively locked using fcntl.lockf. When it is to be released it is removed by default. However, if all cooperating processes turn off the removal, they get a guaranteed order of acquisitions and better scalability.

Class LockError

Parent class: Exception

Raised when a lock file acquisition is unsuccessful because the lock file is held by another process.

__init__(self, message)

Initializes the exception.

__repr__(self)

Returns the official string representation.

Class LockFile

Parent class: object

This class represents an acquired lock file. After its releasing most methods lose their sense and raise a ValueError.

__init__(self, path, wait = False, remove = True)

Creates and locks the specified file. The wait argument can be set to True to wait until the lock file can be acquired. The remove argument can be set to False to keep the file after releasing.

Raises LockError if the wait argument is False and the lock file is held by another process. Raises OSError or IOError if any other error occurs. In particular, raises IOError with the errno attribute set to errno.EINTR, if waiting for the acquisition is interrupted by a signal.

__enter__(self)

Returns self.

Raises ValueError if the lock file is released.

__exit__(self, exc_type, exc_value, traceback)

Calls release.

Raises exceptions raised by release.

__repr__(self)

Returns the official string representation.

fileno(self)

Returns the file descriptor used by the lock file.

Raises ValueError if the lock file is released.

get_path(self)

Returns the path of the lock file.

Raises ValueError if the lock file is released.

release(self)

Removes (optionally) and closes the lock file.

Raises ValueError if the lock file is already released. Raises OSError if any other error occurs.

Examples

Checking if at most one instance of the script is running:

import sys
from lock_file import LockFile, LockError

try:
    lock_f = LockFile('/var/run/app.lock')
except LockError:
    sys.exit('The script is already running')

try:
    do_something_useful()
finally:
    lock_f.release()

Waiting for the lock file acquisition:

from lock_file import LockFile

lock_f = LockFile('/var/run/app.lock', wait = True)
try:
    do_something_useful()
finally:
    lock_f.release()

Waiting for the lock file acquisition (in Python 2.5 and higher):

from __future__ import with_statement
from lock_file import LockFile

with LockFile('/var/run/app.lock', wait = True):
    do_something_useful()