On 2011-06-29, steve+comp.lang.pyt...@pearwood.info
<steve+comp.lang.pyt...@pearwood.info> wrote:
> 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?
That's how Unix programs have modified files "in place" since time
immemorial.
> Is there a better/safer way?
Many programs rename the original file with a "backup" suffix (a tilde
is popular).
--
Grant Edwards grant.b.edwards Yow! It's NO USE ... I've
at gone to "CLUB MED"!!
gmail.com
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