Question

I’m working on a python script that will be accessed via the web, so there will be multiple users trying to append to the same file at the same time. My worry is that this might cause a race condition where if multiple users wrote to the same file at the same time and it just might corrupt the file.

For example:

#!/usr/bin/env python

g = open("/somepath/somefile.txt", "a")
new_entry = "foobar"
g.write(new_entry)
g.close

Will I have to use a lockfile for this as this operation looks risky.

Answer 1

You can use file locking:

import fcntl
new_entry = "foobar"
with open("/somepath/somefile.txt", "a") as g:
    fcntl.flock(g, fcntl.LOCK_EX)
    g.write(new_entry)
    fcntl.flock(g, fcntl.LOCK_UN)

Note that on some systems, locking is not needed if you’re only writing small buffers, because appends on these systems are atomic.