Python Cookbook

Using the cPickle Module on Classes and Instances

Credit: Luther Blissett

Problem

You want to save and restore class and instance objects using the cPickle module.

Solution

Often, you need no special precautions to use cPickle on your classes and their instances. For example, the following works fine:

import cPickle

class ForExample:
    def _ _init_ _(self, *stuff): self.stuff = stuff
anInstance = ForExample('one', 2, 3)
saved = cPickle.dumps(anInstance)
reloaded = cPickle.loads(saved)
assert anInstance.stuff == reloaded.stuff

However, sometimes there are problems:

anotherInstance = ForExample(1, 2, open('three', 'w'))
wontWork = cPickle.dumps(anotherInstance)

This causes a TypeError: “can’t pickle file objects exception”, because the state of anotherInstance includes a file object, and file objects cannot be pickled. You would get exactly the same exception if you tried to pickle any other container that includes a file object among its items.

However, in some cases, you may be able to do something about it:

import types
class PrettyClever:
    def _ _init_ _(self, *stuff): self.stuff = stuff
    def _ _getstate_ _(self):
        def normalize(x):
            if type(x) == types.FileType:
                return 1, (x.name, x.mode, x.tell(  ))
            return 0, x
        return [ normalize(x) for x in self.stuff ]
    def _ _setstate_ _(self, stuff):
        def reconstruct(x):
            if x[0] == 0:
                return x[1]
            name, mode, offs = x[1]
            openfile = open(name, mode)
            openfile.seek(offs)
            return openfile
        self.stuff = tuple([reconstruct(x) for x in stuff])

By defining the _ _getstate_ ...