Unable to load files using pickle and multiple modules

The issue is that you're pickling objects defined in Settings by actually running the 'Settings' module, then you're trying to unpickle the objects from the GUI module.

Remember that pickle doesn't actually store information about how a class/object is constructed, and needs access to the class when unpickling. See wiki on using Pickle for more details.

In the pkl data, you see that the object being referenced is __main__.Manager, as the 'Settings' module was main when you created the pickle file (i.e. you ran the 'Settings' module as the main script to invoke the addUser function).

Then, you try unpickling in 'Gui' - so that module has the name __main__, and you're importing Setting within that module. So of course the Manager class will actually be Settings.Manager. But the pkl file doesn't know this, and looks for the Manager class within __main__, and throws an AttributeError because it doesn't exist (Settings.Manager does, but __main__.Manager doesn't).

Here's a minimal code set to demonstrate.

The class_def.py module:

import pickle

class Foo(object):
    def __init__(self, name):
        self.name = name

def main():
    foo = Foo('a')
    with open('test_data.pkl', 'wb') as f:
        pickle.dump([foo], f, -1)

if __name__=='__main__':
    main()

You run the above to generate the pickle data. The main_module.py module:

import pickle

import class_def

if __name__=='__main__':
    with open('test_data.pkl', 'rb') as f:
        users = pickle.load(f)

You run the above to attempt to open the pickle file, and this throws roughly the same error that you were seeing. (Slightly different, but I'm guessing that's because I'm on Python 2.7)

The solution is either:

1. You make the class available within the namespace of the top-level module (i.e. GUI or main_module) through an explicit import, or
2. You create the pickle file from the same top-level module as the one that you will open it in (i.e. call Settings.addUser from GUI, or class_def.main from main_module). This means that the pkl file will save the objects as Settings.Manager or class_def.Foo, which can then be found in the GUI`main_module` namespace.

Option 1 example:

import pickle

import class_def
from class_def import Foo # Import Foo into main_module's namespace explicitly

if __name__=='__main__':
    with open('test_data.pkl', 'rb') as f:
        users = pickle.load(f)

Option 2 example:

import pickle

import class_def

if __name__=='__main__':
    class_def.main() # Objects are being pickled with main_module as the top-level
    with open('test_data.pkl', 'rb') as f:
        users = pickle.load(f)

Please first read the answer mentioned by zehnpaard to know the reason for the attribute error. Other than the solution he already provided, in python3 you can use the pickle.Unpickler class and override the find_class method as mentioned below:

import pickle

class CustomUnpickler(pickle.Unpickler):

    def find_class(self, module, name):
        if name == 'Manager':
            from settings import Manager
            return Manager
        return super().find_class(module, name)

pickle_data = CustomUnpickler(open('file_path.pkl', 'rb')).load()

If you're still getting this error even after importing the appropriate classes in the loading module (zehnpaard's solution #1), then the find_class function of pickle.Unpickler can be overwritten and explicitly directed to look in the current module's namespace.

import pickle
from settings import Manager

class CustomUnpickler(pickle.Unpickler):

    def find_class(self, module, name):
        try:
            return super().find_class(__name__, name)
        except AttributeError:
            return super().find_class(module, name)

pickle_data = CustomUnpickler(open('file_path.pkl', 'rb')).load()
## No exception trying to get 'Manager'

Note: This method loses the relative-import path information stored in module. So, be careful of namespace collisions in your pickled classes.