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 -- http://mail.python.org/mailman/listinfo/python-list
