On Mon, 30 Jan 2017 09:39 pm, Peter Otten wrote:
>>>> def rename(source, dest):
> ... os.link(source, dest)
> ... os.unlink(source)
> ...
>>>> rename("foo", "baz")
>>>> os.listdir()
> ['bar', 'baz']
>>>> rename("bar", "baz")
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> File "<stdin>", line 2, in rename
> FileExistsError: [Errno 17] File exists: 'bar' -> 'baz'
Thanks Peter!
That's not quite ideal, as it isn't atomic: it is possible that the link
will succeed, but the unlink won't. But I prefer that over the alternative,
which is over-writing a file and causing data loss.
So to summarise, os.rename(source, destination):
- is atomic on POSIX systems, if source and destination are both on the
same file system;
- may not be atomic on Windows?
- may over-write an existing destination on POSIX systems, but not on
Windows;
- and it doesn't work across file systems.
os.replace(source, destination) is similar, except that it may over-write an
existing destination on Windows as well as on POSIX systems.
The link/unlink trick:
- avoids over-writing existing files on POSIX systems at least;
- but maybe not Windows?
- isn't atomic, so in the worst case you end up with two links to
the one file;
- but os.link may not be available on all platforms;
- and it won't work across file systems.
Putting that all together, here's my attempt at a version of file rename
which doesn't over-write existing files:
import os
import shutil
def rename(src, dest):
"""Rename src to dest only if dest doesn't already exist (almost)."""
if hasattr(os, 'link'):
try:
os.link(src, dest)
except OSError:
pass
else:
os.unlink(src)
return
# Fallback to an implementation which is vulnerable to a
# Time Of Check to Time Of Use bug.
# Try to reduce the window for this race condition by minimizing
# the number of lookups needed between one call and the next.
move = shutil.move
if not os.file.exists(dest):
move(src, dest)
else:
raise shutil.Error("Destination path '%s' already exists" % dest)
Any comments? Any bugs? Any cross-platform way to slay this TOCTOU bug once
and for all?
--
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.
--
https://mail.python.org/mailman/listinfo/python-list
