how to use 'pickle'.

Question

my code(i was unable to use 'pickle'):

class A(object):
    def __getstate__(self):
        print 'www'
        return 'sss'
    def __setstate__(self,d):
        print 'aaaa'

import pickle
a = A()
s = pickle.dumps(a)
e = pickle.loads(s)
print s,e

print :

www
aaaa
ccopy_reg
_reconstructor
p0
(c__main__
A
p1
c__builtin__
object
p2
Ntp3
Rp4
S'sss'
p5
b. <__main__.A object at 0x00B08CF0>

who can tell me how to use.

Answer 1

In a nutshell, in your example, e equals a.

Don't have to care about these strang strings, you can dumps these strings to save to anywhere, just remember when you loads them, you got 'a' object again.

Answer 2

You are able to pickle (meaning, this code works as it should). You just seem to get a result, you don't expect. If you expect the same 'output', try:

import pickle
a = A()
s = pickle.dumps(a)
e = pickle.loads(s)
print s, pickle.dumps(e)

Your example isn't, well, a typical 'pickling' example. Usually pickled objects are saved somewhere persistently or sent over the wire. See e.g. pickletest.py: http://www.sthurlow.com/python/lesson10/.

There are advanced uses of pickling, see for example David Mertz XML object serialisation article: http://www.ibm.com/developerworks/xml/library/x-matters11.html

Answer 3

What are you trying to do? It works for me:

class A(object):
    def __init__(self):
        self.val = 100

    def __str__(self):
        """What a looks like if your print it"""
        return 'A:'+str(self.val)

import pickle
a = A()
a_pickled = pickle.dumps(a)
a.val = 200
a2 = pickle.loads(a_pickled)
print 'the original a'
print a
print # newline
print 'a2 - a clone of a before we changed the value'
print a2
print 

print 'Why are you trying to use __setstate__, not __init__?'
print

So this will print:

the original a
A:200

a2 - a clone of a before we changed the value
A:100

If you need setstate:

class B(object):
    def __init__(self):
        print 'Perhaps __init__ must not happen twice?'
        print
        self.val = 100

    def __str__(self):
        """What a looks like if your print it"""
        return 'B:'+str(self.val)

    def __getstate__(self):
        return self.val

    def __setstate__(self,val):
        self.val = val

b = B()
b_pickled = pickle.dumps(b)
b.val = 200
b2 = pickle.loads(b_pickled)
print 'the original b'
print b
print # newline
print 'b2 - b clone of b before we changed the value'
print b2

which prints:

Why are you trying to use __setstate__, not __init__?

Perhaps __init__ must not happen twice?

the original b
B:200

b2 - b clone of b before we changed the value
B:100