Safely renaming a file without overwriting

Diez B. Roggisch deets at nospam.web.de
Sat Oct 28 16:48:37 CEST 2006


Wolfgang Draxinger schrieb:
> Steven D'Aprano wrote:
> 
>> I want to rename a file, but only if the destination file name
>> doesn't already exist.
>>
>> I can do this:
>>
>> if os.path.exists(dest):
>>     # skip file, raise an exception, make a backup...
>>     do_something_else()
>> else:
>>     os.rename(src, dest)
>>
>>
>> But on a multi-user system, it is possible that dest is created
>> in the time period between checking if it exists and attempting
>> the rename.
>>
>> Is there any way to prevent this? Or do I just try to keep the
>> check and the rename as close together as possible, minimizing
>> the chances and hoping for the best?
> 
> 1: Open the file with os.open
> 
> 2: Lock the file exclusively -> no other process can now access
> it.
> 
> 3: Use rename to rename the file; this causes a file system level
> implicit unlink of the old file (it dissappears from the file
> system) but the opening process can still access it.
> 
> 4: close the file -> the lock is removed and the rename
> finalized.

Where does that help for new files? The OP was right in assuming that a 
race condition could occur when he tests for a file & then tries to 
create it, as in the meantime it could have been created.

Diez



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