Re: Pickling and unpickling inherited attributes
- From: aleaxit@xxxxxxxxx (Alex Martelli)
- Date: Sun, 30 Oct 2005 17:25:25 -0700
Alex <OurLab@xxxxxxxxx> wrote:
> I have a serious problem and I hope there is some solution. It is
> easier to illustrate with a simple code:
>
> >>> class Parent(object):
> __slots__=['A', 'B']
> def __init__(self, a, b):
> self.A=a; self.B=b
> def __getstate__(self):
> return self.A, self.B
> def __setstate__(self, tup):
> self.A, self.B=tup
>
>
> >>> class Child(Parent):
> __slots__=['C',]
> def __init__(self, c):
> self.C=c
> def __getstate__(self):
> return self.C,
> def __setstate__(self, tup):
> self.C, =tup
Child.__getstate__ and __setstate__ need to cooperate with those of
Parent, not just override them as yours do. __getstate__ is easy:
in Child:
def __getstate__(self):
return super(Child, self).__getstate__ + (self.C,)
i.e., each class will successively append its parts of state to the
resulting overall tuple. It's harder to do for __setstate__ without
building into it a strong dependency on how many items of the tuple the
Parent is taking for its own use; you need to establish some protocol of
your own for that purpose, such as:
in Parent:
def __setstate__(self, tup):
self.A, self.B = tup[:2]
self._tup = tup[2:]
in Child:
def __setstate__(self, tup):
super(Child, self).__setstate__(tup)
self.C, = self._tup[:1]
self._tup = self._tup[1:]
(the last statement is needed in case Child was further subclassed by a
Grandchild class following the same set-state protocol).
Working with lists rather than tuples might in fact be simpler. But
simplest might be if __setstate__ was allowed to return something
different than None, so it could return "whatever's left of the tuple"
rather than relying on that nasty self._tup thingy (which is going to be
left there, empty, in the end -- though, each class could conditionally
delete it if it was empty). However, while I believe it is not
currently enforced that __setstate__ might return a non-None value to be
ignored, I'm not sure it's _allowed_, either, so I wouldn't try (lest
this application should break in strange ways in future Python
versions).
Alex
.
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