python - AttributeError when unpickling an object

Question

I'm trying to pickle an instance of a class in one module, and unpickle it in another.

Here's where I pickle:

import cPickle

def pickleObject():
    object = Foo()
    savefile = open('path/to/file', 'w')
    cPickle.dump(object, savefile, cPickle.HIGHEST_PROTOCOL)


class Foo(object):
    (...)

and here's where I try to unpickle:

savefile = open('path/to/file', 'r')
object = cPickle.load(savefile)

On that second line, I get AttributeError: 'module' object has no attribute 'Foo'

Anyone see what I'm doing wrong?

Answer 1

class Foo must be importable via the same path in the unpickling environment so that the pickled object can be reinstantiated.

I think your issue is that you define Foo in the module that you are executing as main (__name__ == "__main__"). Pickle will serialize the path (not the class object/definition!!!) to Foo as being in the main module. Foo is not an attribute of the main unpickle script.

In this example, you could redefine class Foo in the unpickling script and it should unpickle just fine. But the intention is really to have a common library that is shared between the two scripts that will be available by the same path. Example: define Foo in foo.py

Simple Example:

$PROJECT_DIR/foo.py

class Foo(object):
    pass

$PROJECT_DIR/picklefoo.py

import cPickle
from foo import Foo

def pickleObject():
    obj = Foo()
    savefile = open('pickle.txt', 'w')
    cPickle.dump(obj, savefile, cPickle.HIGHEST_PROTOCOL)


pickleObject()

$PROJECT_DIR/unpicklefoo.py

import cPickle

savefile = open('pickle.txt', 'r')
obj = cPickle.load(savefile)
...