Make Python Objects JSON serializable

When you're developing a service which has to communicate with a REST endpoint, it is very likely that it will come in handy if you can convert some of your objects to JSON (and back). Until recently, I always created a dictionary and used json.dumps in that case. The same applied when I wanted to log an object to AWS CloudWatch, which has structured logs (JSON logs).

Instead of doing it completely manually, you should use a pattern described by jaraco (and fixed by me):

class MyCustom(object):
    def __json__(self):
        return {
            "a": self.a,
            "b": self.b,
            "__python__": "mymodule.submodule:MyCustom.from_json",
        }

    for_json = __json__  # supported by simplejson

    @classmethod
    def from_json(cls, json):
        obj = cls()
        obj.a = json["a"]
        obj.b = json["b"]
        return obj

With that, the following workflow is possible:

import simplejson

obj = MyCustom()
obj.a = 3
obj.b = 4

json = simplejson.dumps(obj, for_json=True)

# Two-step loading
obj2_dict = simplejson.loads(json)
obj2 = MyCustom.from_json(obj2_dict)

# Make sure we have the correct thing
assert isinstance(obj2, MyCustom)
assert obj2.__dict__ == obj.__dict__

Note that we need two steps for loading. For now, the __python__ property is not used.