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> This sounds silly to me. You can write a file in two lines:
>
> with open("foo", "wb") as f:
> f.write(contents)
If the disk is full, write fails and the new file only contains a part of 'contents'. If the file does already exist and the write fails, the original content is lost.
The correct pattern is something like:
@contextlib.contextmanager
def atomic_write(filename):
 tempname = filename + ".tmp"
 out = open(tempname, "w")
 try:
 yield out
 if hasattr('os', 'fsync'):
 os.fsync(out.fileno())
 out.close()
 if os.name in ('nt', 'ce'):
 os.unlink(filename)
 # ... hope that it doesn't fail here ...
 os.rename(tempname, filename)
 except:
 out.close()
 os.unlink(tempname)
 raise
Remarks:
 - call fsync() to ensure that the new file content is written on disk. it does nothing on Mac OS X
 - on Windows, it's not possible to rename a to b if b does already exist. New Windows versions has an atomic function: MoveFileTransacted(). Older versions have MoveFileEx(MOVEFILE_REPLACE_EXISTING | MOVEFILE_WRITE_THROUGH) and ReplaceFile()
This context manager is *not* atomic on any OS. It's only atomic on some OS, and it may depends on the kernel version (see recent discussions about ext3/ext4, fsync and write barrier).