PEP 3154 introduced the MEMOIZE opcode which lowered the overhead of memoization compared to the PUT opcodes which were previously used.

We should update pickletools.optimize to remove superfluous uses of this new opcode.

I afraid this is not a feature, but a bug. pickletools.optimize() can produce invalid pickled data when *PUT and MEMOIZE opcodes are used.

>>> import pickle, pickletools
>>> p = b'\x80\x04]\x94(\x8c\x04spamq\x01\x8c\x03ham\x94h\x02e.'
>>> pickle.loads(p)
['spam', 'ham', 'ham']
>>> pickletools.dis(p)
    0: \x80 PROTO      4
    2: ]    EMPTY_LIST
    3: \x94 MEMOIZE
    4: (    MARK
    5: \x8c     SHORT_BINUNICODE 'spam'
   11: q        BINPUT     1
   13: \x8c     SHORT_BINUNICODE 'ham'
   18: \x94     MEMOIZE
   19: h        BINGET     2
   21: e        APPENDS    (MARK at 4)
   22: .    STOP
highest protocol among opcodes = 4
>>> p2 = pickletools.optimize(p)
>>> p2
b'\x80\x04\x95\x13\x00\x00\x00\x00\x00\x00\x00]\x94(\x8c\x04spam\x8c\x03ham\x94h\x02e.'
>>> pickle.loads(p2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 2
>>> pickletools.dis(p2)
    0: \x80 PROTO      4
    2: \x95 FRAME      19
   11: ]    EMPTY_LIST
   12: \x94 MEMOIZE
   13: (    MARK
   14: \x8c     SHORT_BINUNICODE 'spam'
   20: \x8c     SHORT_BINUNICODE 'ham'
   25: \x94     MEMOIZE
   26: h        BINGET     2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/pickletools.py", line 2458, in dis
    raise ValueError(errormsg)
ValueError: memo key 2 has never been stored into

Well, that can only happen if MEMOIZE and PUT are both used together, which won't happen with the Pickler classes we support. The easiest thing to do here is to disable pickletools.optimize on protocol 4.

By the way, I was curious if there were many users of pickletools.optimize(), so I did a search and most matches seem to be copies of the stdlib source tree:

http://code.ohloh.net/search?s=pickletools%20optimize&p=0&pp=0&fl=Python&mp=1&ml=1&me=1&md=1&ff=1&filterChecked=true

Is it _documented_ that MEMOIZE and PUT can't be used together?  If not, it should be documented; and pickletools dis() and optimize() should verify that this restriction is honored in their inputs.

MEMOIZE and PUT can be used together. They just need to not step on each other toes when they write to the memo table. As specified by PEP 3154, the memo index used by MEMOIZE is the number of elements currently in the memo table. This obviously means functions like, pickletools.optimize, have to be more careful when they rewrite pickles.

Ah, I almost forgot! I did implement the verification in pickletools.dis() for MEMOIZE:

http://hg.python.org/cpython/file/2612ea573ff7/Lib/pickletools.py#l2420

Here is a patch which makes pickletools.optimize() to work with the MEMOIZE opcode.

As side effect it also renumbers memoized values, this allows to use short BINPUT and BINGET instead of LONG_BINPUT and LONG_BINGET.

Ping.

Here is a patch which addresses Antoine's comments. Also added dis() output for binary example in comments.

New changeset c49b7acba06f by Serhiy Storchaka in branch '3.4':
Issue #19858:  pickletools.optimize() now aware of the MEMOIZE opcode, can
https://hg.python.org/cpython/rev/c49b7acba06f

New changeset e7dd739b4b4e by Serhiy Storchaka in branch 'default':
Issue #19858:  pickletools.optimize() now aware of the MEMOIZE opcode, can
https://hg.python.org/cpython/rev/e7dd739b4b4e

Many thanks for all your reviews Antoine!