How to use Python's JSONDecoder?

March 8, 2022

For the life of me I haven’t been able to find any example of this for Python.

Say I have a class Person, and I want to serialize and deserialize an object from that class.

class Person:
    def __init__(self, n, b):
        self._name = n
        self._born = b

    @property
    def name(self):
        return self._name

    @property
    def born(self):
        return self._born

    def __str__(self):
        return f"{self.name} was born in {self.born.isoformat()}"

p1 = Person("Baltasar", datetime(1919, 7, 1))

Now, I want to serialize that nice Person object, and I want to support datetime objects. So I wrote a custom serializer:

class JSONCoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, datetime):
            return { "__class__": "datetime",
                     "y": obj.year,
                     "month": obj.month,
                     "d": obj.day,
                     "h": obj.hour,
                     "minute": obj.minute,
                     "s": obj.second }

        return json.JSONEncoder.default(self, obj)

The purpose is to be able to support a couple of types, including datetime.

Now comes the time to deserialize. I had to write a custome deserializer to match the serializer. Here it is:

class JSONDCoder(json.JSONDecoder):
    def __init__(self):
        json.JSONDecoder.__init__(self, object_hook=JSONDCoder.from_dict)

    @staticmethod
    def from_dict(d):
        if d.get("__class__") == "datetime":
            return datetime(d["y"], d["month"], d["d"],
                            d["h"], d["minute"], d["s"])

If I trigger:

coder = JSONCoder()
s = coder.encode(p1.__dict__)
print("Encoded: " + s)

dcoder = JSONDCoder()
d = dcoder.decode(s)
print("Decoded: " + str(d))

I obtain the following output:

Encoded: {"_name": "Baltasar", "_born": {"__class__": "datetime", "y": 1919, "month": 7, "d": 0, "h": 0, "minute": 0, "s": 0}}
Decoded: None
...

The only way to obtain anything apart from None, is to modify the decoder and change it to:

class JSONDCoder(json.JSONDecoder):
    def __init__(self):
        json.JSONDecoder.__init__(self, object_hook=JSONDCoder.from_dict)

    @staticmethod
    def from_dict(d):
        if d.get("__class__") == "datetime":
            return d

Whatever other option makes it to return None, as shown above.

How can I have my custom decoder so it supports datetime types?

>Solution :

You simply forgot to add a return statement at the end of the from_dict method.

class JSONDCoder(json.JSONDecoder):
def __init__(self):
    json.JSONDecoder.__init__(self, object_hook=JSONDCoder.from_dict)

@staticmethod
def from_dict(d):
    if d.get("__class__") == "datetime":
        return datetime(d["y"], d["month"], d["d"],
                        d["h"], d["minute"], d["s"])
    return d

This definition should work, the output for me is the following:

Encoded: {"_name": "Baltasar", "_born": {"__class__": "datetime", "y": 1919, "month": 7, "d": 1, "h": 0, "minute": 0, "s": 0}}
Decoded: {'_name': 'Baltasar', '_born': datetime.datetime(1919, 7, 1, 0, 0)}

Let me know if it was what you were looking for.