pickle/unpickle class which has changed
Peter Otten
__peter__ at web.de
Tue Mar 6 09:28:16 EST 2012
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
More information about the Python-list
mailing list