homepage

In Python 3.2:
>>> pickle.dumps(b'xyz', protocol=2)
b'\x80\x02c__builtin__\nbytes\nq\x00]q\x01(KxKyKze\x85q\x02Rq\x03.'
In Python 2.7:
>>> pickle.loads(b'\x80\x02c__builtin__\nbytes\nq\x00]q\x01(KxKyKze\x85q\x02Rq\x03.')
'[120, 121, 122]'
The problem is that the bytes() constructor argument is a list of ints, which gives a different result when reconstructed under 2.x where bytes is an alias of str:
>>> pickletools.dis(pickle.dumps(b'xyz', protocol=2))
 0: \x80 PROTO 2
 2: c GLOBAL '__builtin__ bytes'
 21: q BINPUT 0
 23: ] EMPTY_LIST
 24: q BINPUT 1
 26: ( MARK
 27: K BININT1 120
 29: K BININT1 121
 31: K BININT1 122
 33: e APPENDS (MARK at 26)
 34: \x85 TUPLE1
 35: q BINPUT 2
 37: R REDUCE
 38: q BINPUT 3
 40: . STOP
highest protocol among opcodes = 2
Bytearray objects use a different trick: they pass a (unicode string, encoding) pair which has the same constructor semantics under 2.x and 3.x. Additionally, such encoding is statistically more efficient: a list of 1-byte ints will take 2 bytes per encoded char, while a latin1-to-utf8 transcoded string (BINUNICODE uses utf-8) will take on average 1.5 bytes per encoded char (assuming a 50% probability of higher-than-127 bytes).
>>> pickletools.dis(pickle.dumps(bytearray(b'xyz'), protocol=2))
 0: \x80 PROTO 2
 2: c GLOBAL '__builtin__ bytearray'
 25: q BINPUT 0
 27: X BINUNICODE 'xyz'
 35: q BINPUT 1
 37: X BINUNICODE 'latin-1'
 49: q BINPUT 2
 51: \x86 TUPLE2
 52: q BINPUT 3
 54: R REDUCE
 55: q BINPUT 4
 57: . STOP
highest protocol among opcodes = 2