Python, serialize an actual object from a class into json

Question

I have a class with a child class. I'm able to serialize it into JSON using jsonpickle library. I'm trying to understand something better though. When I try to use Python's native json library, it says the object is not serializable.

I'm looking at this guide: https://www.programiz.com/python-programming/json

He's showing a class defined like this:

person_dict = {"name": "Bob",
"languages": ["English", "Fench"],
"married": True,
"age": 32
}

Which serializes OK.

print(json.dumps(person_dict))
# outputs "{"name": "Bob", "languages": ["English", "Fench"], "married": true, "age": 32}"

But I'm creating a class and instantiating an object from it.

class Purchase(object):
    def __init__(self, receipt_id, order_id):
        self.receipt_id = receipt_id
        self.order_id = order_id
        self.items = []

test = Purchase("123", "abc")
test.items.append("item1")
test.items.append("item2")

But json.dumps() doesn't work here.

print(json.dumps(test))
# outputs "TypeError: Object of type Purchase is not JSON serializable"

What fundamental am I missing here?

The example is a dict, not a class.

JeffUK
– JeffUK

2021-06-24 14:42:01 +00:00
Commented Jun 24, 2021 at 14:42 — JeffUK
– JeffUK, Commented Jun 24, 2021 at 14:42

Yuri · Accepted Answer · 2021-06-24 15:43:52Z

1

In your object, add a method to serialize in json:

def serialize(self):
    """JSON serializer for objects not serializable by default json code"""

    return json.dumps({'items': self.items, 'receipt_id': self.receipt_id, 'order_id': self.order_id})

and call json.dumps with the default parameter:

json.dumps(test, default=Purchase.serialize)

reference: https://docs.python.org/3/library/json.html#json.dumps

If specified, default should be a function that gets called for objects that can’t otherwise be serialized. It should return a JSON encodable version of the object or raise a TypeError. If not specified, TypeError is raised.

>>> json.dumps(test, default=Purchase.serialize)
'"{\\"items\\": [\\"item1\\", \\"item2\\"], \\"receipt_id\\": \\"123\\", \\"order_id\\": \\"abc\\"}"'
>>> d = json.loads('"{\\"items\\": [\\"item1\\", \\"item2\\"], \\"receipt_id\\": \\"123\\", \\"order_id\\": \\"abc\\"}"')
>>> d
'{"items": ["item1", "item2"], "receipt_id": "123", "order_id": "abc"}'

edited Jun 24, 2021 at 15:43

answered Jun 24, 2021 at 14:42

Yuri

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1 Comment

BenW Over a year ago

This only renders the items list. Do you mean something like these options: pynative.com/make-python-class-json-serializable

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