Locking a file in Python

Update as of June 2024

Nowadays there seem to be a number of robust, cross-platform, actively-maintained solutions to this. A few of the most cited in other answers and comments are:

• filelock
• Portalocker
• oslo.concurrency (if you need more general multi-process synchronization utilities)

Original Answer

Alright, so I ended up going with the code I wrote here, on my website ^{link is dead, view on archive.org} (also available on GitHub). I can use it in the following fashion:

from filelock import FileLock

with FileLock("myfile.txt.lock"):
    # work with the file as it is now locked
    print("Lock acquired.")

There is a cross-platform file locking module here: Portalocker

Although as Kevin says, writing to a file from multiple processes at once is something you want to avoid if at all possible.

If you can shoehorn your problem into a database, you could use SQLite. It supports concurrent access and handles its own locking.

I have been looking at several solutions to do that and my choice has been oslo.concurrency

It's powerful and relatively well documented. It's based on fasteners.

Other solutions:

• Portalocker: requires pywin32, which is an exe installation, so not possible via pip
• fasteners: poorly documented
• lockfile: deprecated
• flufl.lock: NFS-safe file locking for POSIX systems.
• simpleflock : Last update 2013-07
• zc.lockfile : Last update 2016-06 (as of 2017-03)
• lock_file : Last update in 2007-10

I prefer lockfile — Platform-independent file locking