Restore a unified diff

[Nick Allen]

After using ndiff from difflib, the function restore
would return the sequence that generated the delta. Unfortunately,
restore does not do the same for unified_diff. I do not see any similar
function that is intended for unified_diff. Does anyone know how to
"restore" from a unified diff generated delta?

Thanks for all your help in advance.

Best Regards,
Nick


Nick Allen <allen@cse.oh io-state.edu>

[Tim Peters]

Re: Restore a unified diff

[Nick Allen][color=blue]
> After using ndiff from difflib, the function restore
> would return the sequence that generated the delta.[/color]

It can actually reproduce either sequence from which the delta was
generated. But this is really trivial: ndiff was intended to produce
diff output for humans to read, and includes the full text of both
input sequences in "the diff" it generates.
[color=blue]
> Unfortunately, restore does not do the same for unified_diff. I do
> not see any similar function that is intended for unified_diff. Does
> anyone know how to "restore" from a unified diff generated delta?[/color]

That's in general impossible, since unified diffs generally omit most
lines that compared equal to begin with. Unified and context diffs
are, in part, compression gimmicks, showing only what changed plus a
bit of context. ndiff shows everything, so can restore everything
too.

[Mike Meyer]

Re: Restore a unified diff

Tim Peters <tim.peters@gma il.com> writes:
[color=blue]
> [Nick Allen][color=green]
>> Unfortunately, restore does not do the same for unified_diff. I do
>> not see any similar function that is intended for unified_diff. Does
>> anyone know how to "restore" from a unified diff generated delta?[/color]
>
> That's in general impossible, since unified diffs generally omit most
> lines that compared equal to begin with. Unified and context diffs
> are, in part, compression gimmicks, showing only what changed plus a
> bit of context. ndiff shows everything, so can restore everything
> too.[/color]

The unix patch utility seems to do a fine job of handling the unix
unified and context diffs. Unified is the preferred format for the
open source projects I contribute patches to.

Possibly this is some other form of "unified diff" than what you get
from "diff -u" on a Unix system?

<mike
--
Mike Meyer <mwm@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.

[Tim Peters]

Re: Restore a unified diff

[Nick Allen][color=blue][color=green][color=darkred]
>>> Unfortunately, restore does not do the same for unified_diff. I do
>>> not see any similar function that is intended for unified_diff.
>>> Does anyone know how to "restore" from a unified diff generated
>>> delta?[/color][/color][/color]

[Tim Peters][color=blue][color=green]
>> That's in general impossible, since unified diffs generally omit
>> most lines that compared equal to begin with. Unified and
>> context diffs are, in part, compression gimmicks, showing only
>> what changed plus a bit of context. ndiff shows everything, so
>> can restore everything too.[/color][/color]

[Mike Meyer][color=blue]
> The unix patch utility seems to do a fine job of handling the unix
> unified and context diffs.[/color]

Of course it does, but "the diff" isn't the only input to `patch`, you
also need to give `patch` the original source file (or one closely
related to it). `patch` would be deep magic indeed if it could deduce
either the "before" or "after" file from a context or unified diff
*alone*. But both the "before" and "after" files *can* be deduced
from an ndiff diff alone.

This remains a truly trivial observation: ndiff can do this because
the full text of both input files is part of the output it produces.
unified/context diffs cannot generally do this because they don't
generally contain the full text of either input file.