portalocker - Cross-platform locking library

Overview

Portalocker is a library to provide an easy API to file locking.

An important detail to note is that on Linux and Unix systems the locks are advisory by default. By specifying the -o mand option to the mount command it is possible to enable mandatory file locking on Linux. This is generally not recommended however. For more information about the subject:

• https://en.wikipedia.org/wiki/File_locking
• http://stackoverflow.com/questions/39292051/portalocker-does-not-seem-to-lock
• https://stackoverflow.com/questions/12062466/mandatory-file-lock-on-linux

The module is currently maintained by Rick van Hattem <Wolph@wol.ph>. The project resides at https://github.com/WoLpH/portalocker . Bugs and feature requests can be submitted there. Patches are also very welcome.

Redis Locks

This library now features a lock based on Redis which allows for locks across multiple threads, processes and even distributed locks across multiple computers.

It is an extremely reliable Redis lock that is based on pubsub.

As opposed to most Redis locking systems based on key/value pairs, this locking method is based on the pubsub system. The big advantage is that if the connection gets killed due to network issues, crashing processes or otherwise, it will still immediately unlock instead of waiting for a lock timeout.

First make sure you have everything installed correctly:

pip install "portalocker[redis]"

Usage is really easy:

import portalocker

lock = portalocker.RedisLock('some_lock_channel_name')

with lock:
    print('do something here')

The API is essentially identical to the other Lock classes so in addition to the with statement you can also use lock.acquire(...).

Python 2

Python 2 was supported in versions before Portalocker 2.0. If you are still using Python 2, you can run this to install:

pip install "portalocker<2"

Tips

On some networked filesystems it might be needed to force a os.fsync() before closing the file so it's actually written before another client reads the file. Effectively this comes down to:

with portalocker.Lock('some_file', 'rb+', timeout=60) as fh:
    # do what you need to do
    ...

    # flush and sync to filesystem
    fh.flush()
    os.fsync(fh.fileno())

Links

• Documentation

  • http://portalocker.readthedocs.org/en/latest/

• Source

  • https://github.com/WoLpH/portalocker

• Bug reports

  • https://github.com/WoLpH/portalocker/issues

• Package homepage

  • https://pypi.python.org/pypi/portalocker

• My blog

  • http://w.wol.ph/

Examples

To make sure your cache generation scripts don't race, use the Lock class:

>>> import portalocker
>>> with portalocker.Lock('somefile', timeout=1) as fh:
...     print >>fh, 'writing some stuff to my cache...'

To customize the opening and locking a manual approach is also possible:

>>> import portalocker
>>> file = open('somefile', 'r+')
>>> portalocker.lock(file, portalocker.EXCLUSIVE)
>>> file.seek(12)
>>> file.write('foo')
>>> file.close()

Explicitly unlocking is not needed in most cases but omitting it has been known to cause issues:

>>> import portalocker
>>> with portalocker.Lock('somefile', timeout=1) as fh:
...     print >>fh, 'writing some stuff to my cache...'

To customize the opening and locking a manual approach is also possible:

>>> import portalocker
>>> file = open('somefile', 'r+')
>>> portalocker.lock(file, portalocker.EXCLUSIVE)
>>> file.seek(12)
>>> file.write('foo')
>>> file.close()

Explicitly unlocking is not needed in most cases but omitting it has been known to cause issues:

>>> import portalocker
>>> with portalocker.Lock('somefile', timeout=1) as fh:
...     print >>fh, 'writing some stuff to my cache...'

To customize the opening and locking a manual approach is also possible:

>>> import portalocker
>>> file = open('somefile', 'r+')
>>> portalocker.lock(file, portalocker.LOCK_EX)
>>> file.seek(12)
>>> file.write('foo')
>>> file.close()

Explicitly unlocking is not needed in most cases but omitting it has been known to cause issues: AzureAD/microsoft-authentication-extensions-for-python#42 (comment)

If needed, it can be done through:

>>> portalocker.unlock(file)

Do note that your data might still be in a buffer so it is possible that your data is not available until you flush() or close().

To create a cross platform bounded semaphore across multiple processes you can use the BoundedSemaphore class which functions somewhat similar to threading.BoundedSemaphore:

>>> import portalocker
>>> n = 2
>>> timeout = 0.1

>>> semaphore_a = portalocker.BoundedSemaphore(n, timeout=timeout)
>>> semaphore_b = portalocker.BoundedSemaphore(n, timeout=timeout)
>>> semaphore_c = portalocker.BoundedSemaphore(n, timeout=timeout)

>>> semaphore_a.acquire()
<portalocker.utils.Lock object at ...>
>>> semaphore_b.acquire()
<portalocker.utils.Lock object at ...>
>>> semaphore_c.acquire()
Traceback (most recent call last):
  ...
portalocker.exceptions.AlreadyLocked

More examples can be found in the tests.

Changelog

Every release has a git tag with a commit message for the tag explaining what was added and/or changed. The list of tags/releases including the commit messages can be found here: https://github.com/WoLpH/portalocker/releases

License

See the LICENSE file.