Convert class object into Json (with datetime field) - TypeError: Object of type datetime is not JSON serializable

You can use check if the value is a date with isinstance(v, datetime.date):

import json
import datetime
from types import SimpleNamespace


class Outcome:
    def __init__(self, name='', date=datetime.datetime.now(), price=0.0):
        self.name = name
        self.date = datetime.datetime.strptime(date, '%Y-%m-%d')
        self.price = float(price)

# convert from json to outcome
txt = b'{"name": "milk", "date": "2020-01-02", "price": 1.99}'
txt = txt.decode("utf-8")
x = json.loads(txt, object_hook=lambda d: SimpleNamespace(**d))
o = Outcome(x.name, x.date, x.price)
print(o.name, o.date, o.price)

# dict comprehension to check if each value is a date
o2 = json.dumps({k: v.isoformat() if isinstance(v, datetime.date) else v
                 for k, v in o.__dict__.items()})

print(o2)

Output:

milk 2020-01-02 00:00:00 1.99
{"name": "milk", "date": "2020-01-02T00:00:00", "price": 1.99}

You can also make an encode() and decode() method in your class to customize the saving and loading of your class to json and use that instead of the lambda fo object_hook