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To: python-list@python.org
From: Neal Becker <ndbecker2@gmail.com>
Subject: Re: pickle/unpickle class which has changed
Date: Tue, 06 Mar 2012 11:29:16 -0500
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Peter Otten wrote:

> Steven D'Aprano wrote:
> 
>> On Tue, 06 Mar 2012 07:34:34 -0500, Neal Becker wrote:
>> 
>>> What happens if I pickle a class, and later unpickle it where the class
>>> now has added some new attributes?
>> 
>> Why don't you try it?
>> 
>> py> import pickle
>> py> class C:
>> ...     a = 23
>> ...
>> py> c = C()
>> py> pickled = pickle.dumps(c)
>> py> C.b = 42  # add a new class attribute
>> py> d = pickle.loads(pickled)
>> py> d.a
>> 23
>> py> d.b
>> 42
>> 
>> 
>> Unless you mean something different from this, adding attributes to the
>> class is perfectly fine.
>> 
>> But... why are you dynamically adding attributes to the class? Isn't that
>> rather unusual?
> 
> The way I understand the problem is that an apparently backwards-compatible
> change like adding a third dimension to a point with an obvious default
> breaks when you restore an "old" instance in a script with the "new"
> implementation:
> 
>>>> import pickle
>>>> class P(object):
> ...     def __init__(self, x, y):
> ...             self.x = x
> ...             self.y = y
> ...     def r2(self):
> ...             return self.x*self.x + self.y*self.y
> ...
>>>> p = P(2, 3)
>>>> p.r2()
> 13
>>>> s = pickle.dumps(p)
>>>> class P(object):
> ...     def __init__(self, x, y, z=0):
> ...             self.x = x
> ...             self.y = y
> ...             self.z = z
> ...     def r2(self):
> ...             return self.x*self.x + self.y*self.y + self.z*self.z
> ...
>>>> p = P(2, 3)
>>>> p.r2()
> 13
>>>> pickle.loads(s).r2()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "<stdin>", line 7, in r2
> AttributeError: 'P' object has no attribute 'z'
> 
> By default pickle doesn't invoke __init__() and updates __dict__ directly.
> As pointed out in my previous post one way to fix the problem is to
> implement a __setstate__() method:
> 
>>>> class P(object):
> ...     def __init__(self, x, y, z=0):
> ...             self.x = x
> ...             self.y = y
> ...             self.z = z
> ...     def r2(self):
> ...             return self.x*self.x + self.y*self.y + self.z*self.z
> ...     def __setstate__(self, state):
> ...             self.__dict__["z"] = 42 # stupid default
> ...             self.__dict__.update(state)
> ...
>>>> pickle.loads(s).r2()
> 1777
> 
> This keeps working with pickles of the new implementation of P:
> 
>>>> q = P(3, 4, 5)
>>>> pickle.loads(pickle.dumps(q)).r2()
> 50

So if in my new class definition there are now some new attributes, and if I did 
not add a __setstate__ to set the new attributes, I guess then when unpickled 
the instance of the class will simply lack those attributes?