On 06/03/2019 16:14, duncan smith wrote: > Hello, > I've been trying to figure out why one of my classes can be > pickled but not unpickled. (I realise the problem is probably with the > pickling, but I get the error when I attempt to unpickle.) > > A relatively minimal example is pasted below. > > >>>> import pickle >>>> class test(dict): > def __init__(self, keys, shape=None): > self.shape = shape > for key in keys: > self[key] = None > > def __setitem__(self, key, val): > print (self.shape) > dict.__setitem__(self, key, val) > > >>>> x = test([1,2,3]) > None > None > None >>>> s = pickle.dumps(x) >>>> y = pickle.loads(s) > Traceback (most recent call last): > File "<pyshell#114>", line 1, in <module> > y = pickle.loads(s) > File "<pyshell#111>", line 8, in __setitem__ > print (self.shape) > AttributeError: 'test' object has no attribute 'shape' > > > I have DUCkDuckGo'ed the issue and have tinkered with __getnewargs__ and > __getnewargs_ex__ without being able to figure out exactly what's going > on. If I comment out the print call, then it seems to be fine. I'd > appreciate any pointers to the underlying problem. I have one or two > other things I can do to try to isolate the issue further, but I think > the example is perhaps small enough that someone in the know could spot > the problem at a glance. Cheers. > > Duncan >
OK, this seems to be a "won't fix" bug dating back to 2003 (https://bugs.python.org/issue826897). The workaround, class DictPlus(dict): def __init__(self, *args, **kwargs): self.extra_thing = ExtraThingClass() dict.__init__(self, *args, **kwargs) def __setitem__(self, k, v): try: do_something_with(self.extra_thing, k, v) except AttributeError: self.extra_thing = ExtraThingClass() do_something_with(self.extra_thing, k, v) dict.__setitem__(self, k, v) def __setstate__(self, adict): pass doesn't work around the problem for me because I need the actual value of self.shape from the original instance. But I only need it for sanity checking, and under the assumption that the original instance was valid, I don't need to do this when unpickling. I haven't managed to find a workaround that exploits that (yet?). Cheers. Duncan -- https://mail.python.org/mailman/listinfo/python-list
