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os.rename on Windows

eryk sun eryksun at gmail.com
Wed Mar 23 10:16:23 EDT 2016 
On Wed, Mar 23, 2016 at 7:17 AM, Steven D'Aprano <steve at pearwood.info> wrote:
>
> According to the documentation, os.rename(original, new) will fail if new
> already exists.

In 3.3+ you can use os.replace. For POSIX systems it's functionally
the same as os.rename. pyosreplace [1] backports os.replace for 2.6,
2.7 and 3.2.

Ignore the comment on the PyPI page that MoveFileEx isn't guaranteed
to be atomic. It won't do a CopyFile because os.replace doesn't use
the flag MOVEFILE_COPY_ALLOWED. For a cross-volume move, which can't
be executed atomically in a single NtSetInformationFile system call,
os.replace fails with the Windows error code ERROR_NOT_SAME_DEVICE
(0x0011 or 17).

[1]: https://pypi.python.org/pypi/pyosreplace

> Would somebody be kind enough to tell me what OSError is raised?

If the file already exists, os.rename fails with winerror set to
ERROR_ALREADY_EXISTS (0x00B7 or 183). If you need to support 2.x, you
should handle WindowsError, which is a subclass of OSError that has
the "winerror" attribute. In 3.3+ OSError itself has the "winerror"
attribute, and in this case WindowsError is just an alias for OSError.
For 3.3+ you can also handle the cross-platform FileExistsError
subclass of OSError.
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