Recently, I reviewed the documentation source for pickle and came
across the following comment:
BAW: Both pickle and cPickle support something called
inst_persistent_id()
which appears to give unknown types a second shot at producing a
persistent
id. Since Jim Fulton can't remember why it was added or what it's
for, I'm
leaving it undocumented.
I couldn't remember this and decided to dig into it and realized that
this was a very useful experimental feature I added way back when
cPickle was in its infancy. This is a fairly useful optimization that
I never got around to evaluating. (Yeah, I know.) It is a hook, like
the persistent_id hook that is called with objects to determine if
they should be pickled by persistent reference. Unlike persistent_id,
it isn't called for built-in types (really types for which pickle has
specific handlers), like strings, numbers, lists, tuples and so on. It
is only called for "instances" (types for which pickle doesn't have
specific handlers). This vastly reduces the number of calls to the
hook. Some tests with ZODB indicated significant improvements in
pickling speed when a hook is used.
If there are no objections, I'll update the Python 2 documentation to
describe this and add a test. The comment above suggests that this
hook is in pickle and cPickle. It is in cPickle, but was removed from
pickle. I propose to add it back to pickle -- mainly for consistency
with cPickle. I'll need to double check how this interacts with the
type dispatching in pickle protocol 2.
Any objections?
Jim
--
Jim Fulton
Zope Corporation
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