The pickle module seems to use string escape characters when pickling; this becomes inefficient e.g. on numpy arrays. Consider the following

z = numpy.zeros(1000, numpy.uint8)
len(z.dumps())
len(cPickle.dumps(z.dumps()))

The lengths are 1133 characters and 4249 characters respectively.

z.dumps() reveals something like "\x00\x00" (actual zeros in string), but pickle seems to be using the string's repr() function, yielding "'\x00\x00'" (zeros being ascii zeros).

i.e. ("0" in z.dumps() == False) and ("0" in cPickle.dumps(z.dumps()) == True)