How to write a custom JSON decoder for a complex object?

The encoder/decoder example you reference (here) could be easily extended to allow different types of objects in the JSON input/output.

However, if you just want a simple decoder to match your encoder (only having Edge objects encoded in your JSON file), use this decoder:

class EdgeDecoder(json.JSONDecoder):
    def __init__(self, *args, **kwargs):
        json.JSONDecoder.__init__(self, object_hook=self.object_hook, *args, **kwargs)
    def object_hook(self, dct):
        if 'Actor' in dct:
            actor = Actor(dct['Actor']['Name'], dct['Actor']['Age'], '')
            movie = Movie(dct['Movie']['Title'], dct['Movie']['Gross'], '', dct['Movie']['Year'])
            return Edge(actor, movie)
        return dct

Using the code from the question to define classes Movie, Actor, Edge, and EdgeEncoder, the following code will output a test file, then read it back in:

filename="test.json"
movie = Movie('Python', 'many dollars', '', '2000')
actor = Actor('Casper Van Dien', 49, '')
edge = Edge(actor, movie)
with open(filename, 'w') as jsonfile:
    json.dump(edge, jsonfile, cls=EdgeEncoder)
with open(filename, 'r') as jsonfile:
    edge1 = json.load(jsonfile, cls=EdgeDecoder)
assert edge1 == edge