Jeffrey Barish wrote:
> I have a class derived from string that is used in a pickle. In the new
> version of my program, I moved the module containing the definition of the
> class. Now the unpickle fails because it doesn't find the module. I was
> thinking that I could make the unpickle work by putting a copy of the
> module in the original location and then redefine the class by sticking a
> __setstate__ in the class thusly:
>
> def __setstate__(self, state):
> self.__dict__.update(state)
> self.__class__ = NewClassName
>
> My plan was to specify the new location of the module in NewClassName.
> However, when I do this I get the message "'class' object layout differs
> from 'class'". I don't think that they do as the new module is a copy of
> the old one. I suspect that I am not allowed to make the class assignment
> because my class is derived from string. What is the best way to update
> the pickle? The only thought I have is to read all the data with the old
> class module, store the data in some nonpickle format, and then, with
> another program, read the nonpickle-format file and rewrite the pickle
> with the class module in the new location.
You could overwrite Unpickler.find_class():
import pickle
from cStringIO import StringIO
class AliasUnpickler(pickle.Unpickler):
def __init__(self, aliases, *args, **kw):
pickle.Unpickler.__init__(self, *args, **kw)
self.aliases = aliases
def find_class(self, module, name):
module, name = self.aliases.get((module, name), (module, name))
return pickle.Unpickler.find_class(self, module, name)
def loads(aliases, str):
file = StringIO(str)
return AliasUnpickler(aliases, file).load()
if __name__ == "__main__":
import before, after
data = before.A()
print data.__class__, data
dump = pickle.dumps(data)
data = loads({("before", "A"): ("after", "B")}, dump)
print data.__class__, data
In the example the aliases dictionary maps (module, classname) pairs to
(module, classname) pairs. Of course this only works when the class layout
wasn't changed.
Peter
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