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Currently in pickle with protocol 4 the NEWOBJ_EX opcode is used to reconstruct objects with __getnewargs__() or __getnewargs_ex__() even if there no keyword arguments.
>>> import pickle, pickletools, collections
>>> P = collections.namedtuple('P', 'x y')
>>> pickletools.dis(pickle.dumps(P(12, 34), 3))
 0: \x80 PROTO 3
 2: c GLOBAL '__main__ P'
 14: q BINPUT 0
 16: K BININT1 12
 18: K BININT1 34
 20: \x86 TUPLE2
 21: q BINPUT 1
 23: \x81 NEWOBJ
 24: q BINPUT 2
 26: . STOP
highest protocol among opcodes = 2
>>> pickletools.dis(pickle.dumps(P(12, 34), 4))
 0: \x80 PROTO 4
 2: \x95 FRAME 28
 11: \x8c SHORT_BINUNICODE '__main__'
 21: \x94 MEMOIZE
 22: \x8c SHORT_BINUNICODE 'P'
 25: \x94 MEMOIZE
 26: \x93 STACK_GLOBAL
 27: \x94 MEMOIZE
 28: K BININT1 12
 30: K BININT1 34
 32: \x86 TUPLE2
 33: \x94 MEMOIZE
 34: } EMPTY_DICT
 35: \x94 MEMOIZE
 36: \x92 NEWOBJ_EX
 37: \x94 MEMOIZE
 38: . STOP
highest protocol among opcodes = 4
These EMPTY_DICT//MEMOIZE//NEWOBJ_EX can be replaced by single NEWOBJ. Actually there is a regression in protocol 4 against protocol 3, so may be following patch should be applied on 3.4 too.
Here is a patch which uses __newobj__ instead of __newobj_ex__ if there are no keywords arguments. Also it reduce the number of lines of C code and adds tests for classes with __getnewargs__() and __getnewargs_ex__().

Ping.

Some examples (with issue19858 for protocol 4 optimization).
Unpatched:
>>> len(pickle.dumps([P(12, 34) for i in range(1000)], 3))
17258
>>> len(pickletools.optimize(pickle.dumps([P(12, 34) for i in range(1000)], 3)))
8018
>>> len(pickle.dumps([P(i, -i) for i in range(1000)], 3))
20999
>>> len(pickletools.optimize(pickle.dumps([P(i, -i) for i in range(1000)], 3)))
11759
>>> len(pickle.dumps([P(12, 34) for i in range(1000)], 4))
12031
>>> len(pickletools.optimize(pickle.dumps([P(12, 34) for i in range(1000)], 4)))
9028
>>> len(pickle.dumps([P(i, -i) for i in range(1000)], 4))
15772
>>> len(pickletools.optimize(pickle.dumps([P(i, -i) for i in range(1000)], 4)))
12769
Patched:
>>> len(pickle.dumps([P(12, 34) for i in range(1000)], 4))
10031
>>> len(pickletools.optimize(pickle.dumps([P(12, 34) for i in range(1000)], 4)))
8028
>>> len(pickle.dumps([P(i, -i) for i in range(1000)], 4))
13772
>>> len(pickletools.optimize(pickle.dumps([P(i, -i) for i in range(1000)], 4)))
11769

Updated patch addresses Antoine's comments. Also added few comments explaining binary data.

New changeset 2ffaac4c8e53 by Serhiy Storchaka in branch 'default':
Issue #22783: Pickling now uses the NEWOBJ opcode instead of the NEWOBJ_EX
https://hg.python.org/cpython/rev/2ffaac4c8e53