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 -- http://mail.python.org/mailman/listinfo/python-list
