[Python-Dev] os.rename() behavior (original) (raw)

Guido van Rossum guido at python.org
Wed Mar 10 11:14:01 EST 2004

rename(src, dst) Rename the file or directory src to dst. If dst is a directory, OSError will be raised. On Unix, if dst exists and is a file, it will be removed silently if the user has permission. The operation may fail on some Unix flavors if src and dst are on different filesystems. If successful, the renaming will be an atomic operation (this is a POSIX requirement). On Windows, if dst already exists, OSError will be raised even if it is a file; there may be no way to implement an atomic rename when dst names an existing file. Availability: Macintosh, Unix, Windows.

While I understand that this is an operating system behavior, and the os module is meant to hold incompatible functionality, I do belive that most of us developing in posix systems have never checked if a rename failed because the file already exists. IOW, this will usually be discovered when the program blows up for the first time in Windows (that's how Thomas discovered). Is it too late to implement the forced rename internally, and leave the programmer test it with os.path.exists() if he really wants to? I know this will cause incompatibilities in programs which expect the current behavior. OTOH, programs that expect this behavior are already broken in any posix system.

-1.

The Unix semantics can't be properly emulated on Windows, because it is guaranteed to be atomic. Python can't know whether an applications that blindly assumed Unix semantics is relying on the atomicity; if the application needs this, doing a remove() first under the cover would be a grave error (especially more grave since the potential data loss is unlikely to be found through testing).

--Guido van Rossum (home page: http://www.python.org/~guido/)

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