AttributeError while unpickling: Can't get attribute 'Location'

Question

I'm writing a Python CGI script for an inventory system. It needs to store through pickle a list (called locations) of objects. Here's the code I'm using:

try:
    with open(".config/autosave.bin", "rb") as dataFile:
        locations = pickle.load(dataFile)
except (FileNotFoundError, PermissionError):
    dispHTML("p", contents="Error in load:  Save file incorrectly configured!")
    locations = []

However, this results in:

 /Applications/MAMP/cgi-bin/ic/main.py in ()
     16 try:
     17         with open(".config/autosave.bin", "rb") as dataFile:
=>   18                 locations = pickle.load(dataFile)
     19 except (FileNotFoundError, PermissionError):
     20         dispHTML("p", contents="Error in load:  Save file incorrectly configured!")
AttributeError: Can't get attribute 'Location' on <module '__main__' from '/Applications/MAMP/cgi-bin/ic/main.py'> 
      args = ("Can't get attribute 'Location' on <module '__main__' from '/Applications/MAMP/cgi-bin/ic/main.py'>",) 
      with_traceback = <built-in method with_traceback of AttributeError object>

As you can see the save file is stored at .config/autosave.bin. Writing to it seems to work fine but I haven't been able to check.

How can I fix this?

Answer 1

The pickle reading code requires the definition of the Location class. If not it will not be able to reconstruct custom objects of that class.

# config_writer.py

import pickle

class Location:
    def __init__(self, store, aisle):
        self.store = store
        self.aisle = aisle

locations = [Location(i, i) for i in range(10)]
with open('.config/autosave.bin', 'wb') as f:
    pickle.dump(locations, f)

Here is a example that tries to read the pickle file without having the class definition of Location (run this code in another terminal/session):

>>> import pickle
>>> with open('.config/autosave.bin','rb') as f:
...     data = pickle.load(f)
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
AttributeError: Can't get attribute 'Location' on <module '__main__' (built-in)>

However, with access to the class definition:

>>> from config_writer import Location
>>> with open('.config/autosave.bin','rb') as f:
...     data = pickle.load(f)
>>> data
[<config_writer.Location object at 0x7f8b472111d0>, <config_writer.Location object at 0x7f8b41ad6e48>, <config_writer.Location object at 0x7f8b41adb0f0>, <config_writer.Location object at 0x7f8b41adb128>, <config_writer.Location object at 0x7f8b41adb160>, <config_writer.Location object at 0x7f8b41adb198>, <config_writer.Location object at 0x7f8b41adb1d0>, <config_writer.Location object at 0x7f8b41adb208>, <config_writer.Location object at 0x7f8b41adb240>, <config_writer.Location object at 0x7f8b41adb278>]

Hopefully the code that reads the pickle file is able to import the class definition for Location from some other module as does my example.