Exclusively create a file in Python 3

• Posted 27 March 2016
• Tagged with python

I’ve been tidying up a lot of old Python code recently, and I keep running into this pattern:

if not os.path.exists('newfile.txt'):
    with open('newfile.txt', 'w') as f:
        f.write('hello world')

The program wants to write some text to this file, but only if nobody’s written to it before – they don’t want to overwrite the existing contents. This approach is very sensible: if we check that the file exists before writing, we can avoid scribbling over a pre-existing file.

But this code is subject to a race condition: if the file pops into existence between the if and the open(), we scribble all over it anyway.

To catch this race condition, Python 3.3 added a new file mode: x for exclusive creation. If you open a file in mode x, the file is created and opened for writing – but only if it doesn’t already exist. Otherwise you get a FileExistsError.

Here’s how I’d rewrite the snippet above:

try:
    with open('newfile.txt', 'x') as f:
        f.write('hello world')
except FileExistsError:
    print('File already exists.  Clean up!')

Using the x mode means you can be sure that you won’t override an existing file. It’s safer than the existence check.

I probably won’t use this a lot, but when I do, I’ll appreciate it. This has been my general experience with Python 3: there’s no killer feature that I can’t live without, just a growing pile of small niceties that I miss when I go back to Python 2.