Serializing a list of object instances nested within an object instance python-CodePudding

I'm making an app that would allow you to either log in or create a restaurant. As a restaurant owner, you can add/remove/edit refrigerators. My end goal is that I'd have a list of Restaurants that I'd write to a JSON file, and anytime I rerun the app, I can pull that data in and simulate "being a restaurant owner" and edit the fridges for the chosen restaurant.

I essentially want this:

data = {
    restaurants: [
        {
            restaurant: {
            name: "Peppy",
            pw: "123",
            fridges: [
                {
                    fridge: {
                        owner: restaurant.name,
                        contents: []
                    }
                }
            ]
        }
        }
    ]
}

I have the following two classes(showing relevant methods):


class Restaurant:
    def __init__(self, owner, password):
        self.__password = password
        self.owner = owner
        self.__owned_fridges = [] # list of owned fridges

    def add_fridge(self):
        nickname = input("What would you like to name the fridge: ")
        self.__owned_fridges.append(fr(self.owner, nickname))
        print("Fridge added!")
 

class Fridge:
    def __init__(self, owner, nickname):
        self.nickname = nickname
        self.owner = owner
        self.__authorized_users = [owner]
        self.__contents = []

    def add_to_fridge(self):
        if len(self.__contents) == 5:
            print("Your fridge is full!")
        else:
            item = input("What would you like to add : ")
            self.__contents.append(item)

My issue is in serializing this for JSON. I have found that the following works to serialize the restaurant object to JSON, but not the nested fridge objects :

data = {
    'restaurants': []
}

# Testing code
test = res("Jac", "350b534")
test.add_fridge()
test.add_fridge()
data['restaurants'].append(json.dumps(test.__dict__))

I'm relatively new to python, and I come from a js background, so I'm still getting familiar with the syntax. My question is, how do I serialize the inner list of fridges?

CodePudding user response：

Firstly, I'd suggest checking out dataclasses as they will simplify the task of working with classes in Python. With dataclasses, you don't need to define dunder methods like __init__ and __repr__ as they will be generated automatically by default.

For your particular use case, you can solve the issue with serializing a nested class model to a dict / JSON string using an optional default callable that you can pass in to json.dumps. For instance, you can pass a lambda (which is essentially a shorthand function) such as lambda o: o.__dict__, which retrieves the __dict__ attribute for objects that are not inherently serializable to JSON, as shown below.

from __future__ import annotations

import json
from dataclasses import dataclass


@dataclass
class A:
    my_string: str
    b: list[B]


@dataclass
class B:
    my_int: int
    password: str


a = A('test', [B(2, '123'), B(5, '321')])
print(a)
# A(my_string='test', b=[B(my_int=2, password='123'), B(my_int=5, password='321')])

print(json.dumps(a, default=lambda o: o.__dict__))
# {"my_string": "test", "b": [{"my_int": 2, "password": "123"}, {"my_int": 5, "password": "321"}]}

But note that dataclasses also provides a helper function asdict if you need to convert a dataclass instance to a dict instance first.

Though, if you have a more advanced use case - for example, de-serializing JSON data to a nested class model, or mapping a dataclass field such as password to another JSON key such as pw as in the listed example - I'd suggest checking out a serialization library like the dataclass-wizard, which works well with a nested dataclass model as above.