proposal for a nice enhancement to "bzr diff --using <tool>"

[James Westby]

On Wed, 2008-09-10 at 15:06 +0200, David Ingamells wrote:
> Since my python and bzr internals knowledge is almost zero and 
> absolutely zero respectively, I'll only be able to implement this myself 
> (which I'm willing to do) if someone can point me at the right place in 
> the code.

Looking in bzrlib/builtins.py where the commands are defined
for cmd_diff, and then where the "using" option goes, shows
that it just gets passed through to "show_diff_trees" in
bzrlib/diff.py.

This passes it on to DiffTree.from_trees_options, which has

        if using is not None:
            extra_factories = [DiffFromTool.make_from_diff_tree(using)]

which creates a DiffFromTool for the --using argument. It then passes
extra_factories to it's own __init__, which puts them in self.differs.

Looking to where else this is used brings us to show_diff, which
has

              if changed_content:
                self.diff(file_id, oldpath, newpath)

and looking at diff() shows

        try:
            old_kind = self.old_tree.kind(file_id)
        except (errors.NoSuchId, errors.NoSuchFile):                          
            old_kind = None

and similar for new_kind. This is then passed to
DiffPath._diff_many which loops over the differs
and calls diff() on each.

Looking at the implementation of diff() from DiffFromTool (what
--using created above) shows

        if (old_kind, new_kind) != ('file', 'file'):
            return DiffPath.CANNOT_DIFF

and as old_kind will be "None" for added files it will refuse to
work on them.

I expect the start of the fix would be changing these lines and
making it handle None for either old_kind or new_kind. Presumably
file->symlink etc. still shouldn't be handled by the graphical
diff tool.

I hope this is a start for you.

Thanks,

James