Atomic `ln -sf` in python (symlink overwriting exsting file)

Question

I want create a symlink, overwriting an existing file or symlink if needed.

I've discovered that os.path.exists only returns True for non-broken symlinks, so I'm guessing that any test must also include os.path.lexists.

What is the most atomic way to implement ln -sf in python? (Ie, preventing a file being created by another process between deletion and symlink creation)

---

Differentiation: This question doesn't specify the atomic requirement

Answer 1

This code tries to minimise the possibilities for race conditions:

import os
import tempfile

def symlink_force(target, link_name):
    '''
    Create a symbolic link link_name pointing to target.
    Overwrites link_name if it exists.
    '''

    # os.replace() may fail if files are on different filesystems
    link_dir = os.path.dirname(link_name)

    while True:
        temp_link_name = tempfile.mktemp(dir=link_dir)
        try:
            os.symlink(target, temp_link_name)
            break
        except FileExistsError:
            pass
    try:
        os.replace(temp_link_name, link_name)
    except OSError:  # e.g. permission denied
        os.remove(temp_link_name)
        raise

Note:

1. If the function is interrupted (e.g. computer crashes), an additional random link to the target might exist.

2. An unlikely race condition still remains: the symlink created at the randomly-named temp_link_name could be modified by another process before replacing link_name.

I raised a python issue to highlight the issues of os.symlink() requiring the target not exist.

Credit to Robert Seimer's input.