How to make file creation an atomic operation?

Question

I am using Python to write chunks of text to files in a single operation:

open(file, 'w').write(text)

If the script is interrupted so a file write does not complete I want to have no file rather than a partially complete file. Can this be done?

Answer 1

Write data to a temporary file and when data has been successfully written, rename the file to the correct destination file e.g

f = open(tmpFile, 'w')
f.write(text)
# make sure that all data is on disk
# see http://stackoverflow.com/questions/7433057/is-rename-without-fsync-safe
f.flush()
os.fsync(f.fileno()) 
f.close()

os.rename(tmpFile, myFile)

According to doc http://docs.python.org/library/os.html#os.rename

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

also

The operation may fail on some Unix flavors if src and dst are on different filesystems.

Note:

• It may not be atomic operation if src and dest locations are not on same filesystem

• os.fsync step may be skipped if performance/responsiveness is more important than the data integrity in cases like power failure, system crash etc