Forward and backward compatibility illustration.

backward_compatibility.py

import json
import pickle

class Foo:
    """ An old Foo."""

    def __init__(self, a, b):
        self.a = a
        self.b = b

foo = Foo(1, 2)

# store it using pickle
state = json.dumps(foo.__dict__)
blob = pickle.dumps(state)

class Foo:
    """ A Foo from the future."""

    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

# re-load into newer class
state = json.loads(pickle.loads(blob))
foo = Foo(**state)

forward_compatibility.py

import json
import pickle

class Foo:
    """ A Foo from the future."""

    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

# store it using pickle
foo = Foo(1, 2, 3)
state = json.dumps(foo.__dict__)
blob = pickle.dumps(state)

class Foo:
    """ An old Foo."""

    def __init__(self, a, b):
        self.a = a
        self.b = b

# re-load into older class
state = json.loads(pickle.loads(blob))
foo = Foo(**state)