deleting a line from a file

[Paddy]

On Apr 1, 6:32 pm, Mark Wooding <m... at distorted.org.uk> wrote:
> Paddy <paddy3... at googlemail.com> wrote:
> > Why not use the fileinput modules functionality to iterate over a file
> > in-place,printing just those lines you want?
>
> From the Python 2.5 manual:
>
> : *Optional in-place filtering:* if the keyword argument `INPLACE=1' is
> : passed to `input()' or to the `FileInput' constructor, the file is
> : moved to a backup file and standard output is directed to the input
> : file (if a file of the same name as the backup file already exists, it
> : will be replaced silently).
>
> This behaviour is very dangerous.  If the script fails half-way through,
> it will leave the partially-written file in place, with the `official'
> name.  The change-over is not atomic, breaking other programs attempting
> to read simultaneously with an update.

I've used this methodology all the time both explicitely writing
utilities like that and using 'perl -p -i -e ...'
theoretically there may be problems. In practice I have people
transforming files in their own workarea, and have never been called
to
clear-up after such a failing.
Although theoretically you can describe a method of failure, I take
issue
with you calling it 'very dangerous' without further qualification.

>
> Two almost-simultaneous updates will corrupt the file without a usable
> backup.  The first will back up the input file, and start writing.  A
> second will /replace/ the backup file with the partially-constructed
> output of the first, and then start processing it; but since its input
> is incomplete, it will produce incomplete output.

Ahah - the qualification. The above would be dangerous, but the OP may
find
that his current flow, or a slight modification of it will make
simultaneous
updates unlikely and use of fileinput OK. If not, then other methods
may
have to be employed. It _is_ good to know the threat and do the
analysis though.

- Paddy.