[Python-Dev] Module renaming and pickle mechanisms

[Brett Cannon]

On Sun, May 18, 2008 at 6:14 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> M.-A. Lemburg wrote:
>>
>> Perhaps I have a misunderstanding of the reasoning behind
>> doing the renaming in the 2.x branch, but it appears that
>> the only reason is to get used to the new names. That's a
>> rather low priority argument in comparison to the breakage
>> the renaming will cause in the 2.x branch.
>
> I think this is the key point here. The possibility of breaking pickling
> compatibility never came up during the PEP 3108 discussions, so wasn't taken
> into account in deciding whether or not backporting the name changes was a
> good idea.
>
> I think it's pretty clear that the code needs to be moved back into the
> modules with the old names for 2.6. The only question is whether or not we
> put any effort into making the new stdlib organisation usable in 2.x, or
> just rely on 2to3 to fix it (note that the "increasing the common subset"
> argument doesn't really apply, since you can catch the import errors in
> order to try both names).

Problem with this is it makes forward-porting revisions to 3.0 a PITA.
By keeping the module names consistent between the versions merging a
revision is just a matter of ``svnmerge merge`` with the usual
3.0-specific changes. Reverting the modules back to the old name will
make forward-porting much more difficult as I don't think svn keeps
rename information around (and thus map the old name to the new name
in terms of diffs).

Alexandre's idea of teaching pickle the mapping of old names to new
might be the best solution. We could have a flag to pickle that
deactivates the renaming. Otherwise we could bump the pickle version
number so that the new number doesn't do the mapping while the old
versions to the implicit module mapping.

And as Greg and Glpyh have pointed out, this is a problem that might
need to be addressed in the future with some changes to our
serialization method (I have no clue how since I don't deal with
pickle very much).

-Brett