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RE: Unpickling data with classes from dynamic modules

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  RE: Unpickling data with classes from dynamic modules "Prasad, Ramit" <ramit.prasad@jpmorgan.com.dmarc.invalid> - 2013-08-21 19:59 +0000

#52791 — RE: Unpickling data with classes from dynamic modules

Fredrik Tolf wrote:
> 
> Dear list,
> 
> I have a system in which I load modules dynamically every now and then
> (that is, creating a module with types.ModuleType, compiling the code for
> the module and then executing it in the module with exec()), and where I
> would wish to be able to have classes in those modules containing classes
> that could pickled, and then unpickled again.
> 
> The problem with that, currently, is of course two-fold: The dynamically
> loaded module may not be loaded at the time when the unpickling takes
> place, but even if it were, it isn't registered in sys.modules, so the
> unpickler can't find it either way. And, of course, that is detected
> already when pickling.
> 
> Is there any way to fix this, at all?
> 
> I considered trying to create subclasses of the pickler and unpickler that
> pickle a reference to a module loader and data for the particular module
> along with a class that comes from such a module, by overriding
> Pickler.save_global and Unpickler.load_global. Unfortunately, however,
> those functions aren't part of the public interface and can't be
> overridden when the C-pickle module is used instead (which, obviously, is
> what normally happens).
> 
> Is there any way around this without having to modify the pickle module
> itself?
> 
> --
> 
> Fredrik Tolf
> --

I believe rather than subclassing the pickler, you are expected to 
change the behavior from within the class via __getstate__ and __setstate__.

http://docs.python.org/2/library/pickle.html#object.__getstate__

Although, for your use case (loading unknown classes) the section on pickle and 
extension types may be more appropriate.

http://docs.python.org/2/library/pickle.html#pickling-and-unpickling-extension-types

Maybe all you need to add is implementation for obj.__reduce__


~Ramit



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