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portalocker - Cross-platform locking library
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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('writing some stuff to my cache...', file=fh)

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

>>> import portalocker
>>> file = open('somefile', 'r+')
>>> portalocker.lock(file, portalocker.LockFlags.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:
https://github.com/AzureAD/microsoft-authentication-extensions-for-python/issues/42#issuecomment-601108266

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 <http://portalocker.readthedocs.io/en/latest/_modules/tests/tests.html>`_.


Versioning
----------

This library follows `Semantic Versioning <http://semver.org/>`_.


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 <https://github.com/WoLpH/portalocker/blob/develop/LICENSE>`_ file.

