Safely modify a file in place -- am I doing it right?

[steve+comp.lang.python at pearwood.info]

I have a script running under Python 2.5 that needs to modify files in
place. I want to do this with some level of assurance that I won't lose
data. E.g. this is not safe:

def unsafe_modify(filename):
    fp = open(filename, 'r')
    data = modify(fp.read())
    fp.close()
    fp = open(filename, 'w')  # <== original data lost here
    fp.write(fp)
    fp.close()  # <== new data not saved until here

If something goes wrong writing the new data, I've lost the previous
contents.

I have come up with this approach:

import os, tempfile
def safe_modify(filename):
    fp = open(filename, 'r')
    data = modify(fp.read())
    fp.close()
    # Use a temporary file.
    loc = os.path.dirname(filename)
    fd, tmpname = tempfile.mkstemp(dir=loc, text=True)
    # In my real code, I need a proper Python file object, 
    # not just a file descriptor.
    outfile = os.fdopen(fd, 'w')
    outfile.write(data)
    outfile.close()
    # Move the temp file over the original.
    os.rename(tmpname, filename)

os.rename is an atomic operation, at least under Linux and Mac, so if the
move fails, the original file should be untouched.

This seems to work for me, but is this the right way to do it? Is there a
better/safer way?



-- 
Steven