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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