[Python-Dev] Module renaming and pickle mechanisms (original) (raw)

Brett Cannon brett at python.org
Tue May 20 06:44:23 CEST 2008

On Mon, May 19, 2008 at 12:26 PM, Brett Cannon <brett at python.org> wrote:

On Mon, May 19, 2008 at 5:08 AM, M.-A. Lemburg <mal at egenix.com> wrote:

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 ? Don't know, possibly. But I am not about to try to figure out. I don't think that an administrative problem such as forward- porting patches to 3.x warrants breakage in the 2.x branch. That's why I suggested changing pickle to deal with the rename, but obviously I am in the minority in that idea. 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. Never came up. 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 renamedmodules" 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 ? I have started a discussion on the stdlib SIG on how to handle this, so I will defer this discussion to there.

The decision was made (by me) to just revert all of the renames in 2.6. A note will be in the docs stating the rename, but otherwise 2to3 will be relied upon for all transitions from old names to new names.

I have updated the PEP to note about which modules need to be reverted and the new steps to rename a module, and added/re-opened the appropriate issues (all attached to issue 2775).

-Brett

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