Neal Becker wrote: > 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?
I don't know. If you don't trust the demo try it yourself with the actual code you have. Throwing in obj = pickle.load(...) print vars(obj) should help. -- http://mail.python.org/mailman/listinfo/python-list
