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