Moving class used in pickle

[Berthold Höllmann]

Jeffrey Barish <jeff_barish at earthlink.net> writes:

> 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 can fiddle with the file class used reading the pickled file. I.e.
the "read()" method could replace each instance of "foo.myclass" by
"greatnewmodule.mynewclass" could bring you back in the game.

Porting some applications of my from 32 to 64 bit i discovered that my
Numeric int arrays really had to be int32. So i opened the (binary)
pickled files and replaced the occurences of "'l'" by "'i'". Later we
replaced Numeric by numpy. I used the editor approach again to replace
"Numeric" string by "numpy" or "numpy.oldnumeric". A collegue of mine
wrote a small reader class implementing the approach working on the
input stream.

Regards
Berthold
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