[Python-Dev] removing inst_persistent_id from pickle (original) (raw)

Jeremy Hylton jeremy@alum.mit.edu
Wed, 13 Nov 2002 11:16:36 -0500

Pickle and cPickle have an undocumented hook called inst_persistent_id(). When Barry updated the pickle documentation (round about the 2.2 release), no one could figure out what it did. I think we've figured out what it does, finally, but I think we should get rid of it before someone tries to use it.

The pickler has a hook for handling persistent objects. The hook basically allows objects to be pickled by reference to some mechanism external to the pickler. The pickler gets a persistent_id() function that returns the external reference. The unpickler needs a persistent_load() function that returns the object given the reference from persistent_id(). This process is fairly general, although the only use I'm familiar with is ZODB.

The inst_persistent_id() hook seems to be designed for a very special case -- that the persistent_id() function returns an object that is unpicklable. The function is only called when the pickler encounters an object that it hasn't handled via persistent_id() or the dispatch table. The object returned by inst_persistent_id() is always passed to save_pers(), just like persistent_id().

We imagine the intended control flow is:

I don't think this odd case is worth the added complexity, particularly since the hook function probably won't be called for a text-mode pickler.

Anyone object to its removal?

Jeremy