[Python-Dev] undesireable unpickle behavior, proposed fix

"Martin v. Löwis" martin at v.loewis.de
Tue Jan 27 19:43:30 CET 2009

> Interning the strings on unpickling makes the pickles smaller, and at
> least for cPickle actually makes unpickling sequences of many objects
> slightly faster.  I have included proposed patches to cPickle.c and
> pickle.py, and would appreciate any feedback.

Please submit patches always to the bug tracker.

On the proposed change: While it is fairly unintrusive, I would like to
propose a different approach - pickle interned strings special. The
marshal module already uses this approach, and it should extend to
pickle (although it would probably require a new protocol).

On pickling, inspect each string and check whether it is interned. If
so, emit a different code, and record it into the object id dictionary.
On a second occurrence of the string, only pickle a backward reference.
(Alternatively, check whether pickling the same string a second time
would be more compact).

On unpickling, support the new code to intern the result strings;
subsequent references to it will go to the standard backreferencing


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