[Python-Dev] Module renaming and pickle mechanisms

[M.-A. Lemburg]

On 2008-05-18 22:24, Brett Cannon wrote:
> 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).

svnmerge is written in Python, so wouldn't it be possible to add
support for maintaining such renaming to that tool ?

I don't think that an administrative problem such as forward-
porting patches to 3.x warrants breakage in the 2.x branch.

After all, the renaming was approached for Python 3.0 and not
2.6 *because* it introduces major breakage.

AFAIR, the discussion on the stdlib-sig also didn't include the
plan to backport such changes to 2.6. Otherwise, we would have
hashed them out there.

> 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).

It is possible to make pickle aware of the module renames, but
that doesn't solve problems with other forms of serialization
or use of the .__module__ attribute in general.

Why can't we just provide a "from __future__ import renamed_modules"
which then provides all the new name to old name mappings in
some form (e.g. module proxies or whatever) and leave the
existing modules in 2.x untouched ?

-- 
Marc-Andre Lemburg
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