cPickle - sharing pickled objects between scripts and imports

[Dave Angel]

On 06/23/2012 12:13 PM, Peter Otten wrote:
> Rotwang wrote:
>
>> Hi all, I have a module that saves and loads data using cPickle, and
>> I've encountered a problem. Sometimes I want to import the module and
>> use it in the interactive Python interpreter, whereas sometimes I want
>> to run it as a script. But objects that have been pickled by running the
>> module as a script can't be correctly unpickled by the imported module
>> and vice-versa, since how they get pickled depends on whether the
>> module's __name__ is '__main__' or 'mymodule' (say). I've tried to get
>> around this by adding the following to the module, before any calls to
>> cPickle.load:
>>
>> if __name__ == '__main__':
>>      import __main__
>>      def load(f):
>>          p = cPickle.Unpickler(f)
>>          def fg(m, c):
>>              if m == 'mymodule':
>>                  return getattr(__main__, c)
>>              else:
>>                  m = __import__(m, fromlist = [c])
>>                  return getattr(m, c)
>>          p.find_global = fg
>>          return p.load()
>> else:
>>      def load(f):
>>          p = cPickle.Unpickler(f)
>>          def fg(m, c):
>>              if m == '__main__':
>>                  return globals()[c]
>>              else:
>>                  m = __import__(m, fromlist = [c])
>>                  return getattr(m, c)
>>          p.find_global = fg
>>          return p.load()
>> cPickle.load = load
>> del load
>>
>>
>> It seems to work as far as I can tell, but I'll be grateful if anyone
>> knows of any circumstances where it would fail, or can suggest something
>> less hacky. Also, do cPickle.Pickler instances have some attribute
>> corresponding to find_global that lets one determine how instances get
>> pickled? I couldn't find anything about this in the docs.
> if __name__ == "__main__":
>     from mymodule import *
>
> But I think it would be cleaner to move the classes you want to pickle into 
> another module and import that either from your main script or the 
> interpreter. That may also spare you some fun with unexpected isinstance() 
> results.
>
>



I would second the choice to just move the code to a separately loaded
module, and let your script simply consist of an import and a call into
that module.

It can be very dangerous to have the same module imported two different
ways (as __main__ and as mymodule), so i'd avoid anything that came
close to that notion.

Your original problem is probably that you have classes with two leading
underscores, which causes the names to be mangled with the module name. 
You could simply remove one of the underscores for all such names, and
see if the pickle problem goes away.




-- 

DaveA