Create an object from a JSON file in python (using a classmethod)?

Question

I wanna create a classmethod that takes a JSON (dictionary) string and creates an instance of the class it's called on. For example, if I have a class Person the inherits from a class Jsonable with age and name:

class Person(Jsonable):
    def __init__(self, name, age):
        self.name = name
        self.age = age
class Jsonable:
    @classmethod
    def from_json(json_string):
        # do the magic here

and if I have a JSON string string = "{'name': "John", 'age': 21}" and when I say person1 = Person.from_json(string) I want to create person1 with name John and age 21. I also have to keep the class name in some way so that when I call for example Car.from_json(string) it raises a TypeError.

Andrii Rusanov · Accepted Answer · 2016-03-01 11:38:23Z

3

it supposes you have a key __class in you JSON string that holds target class name

import json

class Jsonable(object):
    @classmethod
    def from_json(cls, json_string):
        attributes = json.loads(json_string)
        if not isinstance(attributes, dict) or attributes.pop('__class') != cls.__name__:
            raise ValueError
        return cls(**attributes)

answered Mar 1, 2016 at 11:38

Andrii Rusanov

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3 Comments

bhk Over a year ago

However, say I have the following method in the Jsonable: def to_json(self, indent=4): return json.dumps(self.__dict__, indent) which returns a dictionary with attributes as keys and values as dict values. And I want to use this same string as a parameter in the from_json method, which wouldn't have a __class key.

Andrii Rusanov Over a year ago

I'm afraid it will not contain anything like class name, so you will need to put it there by yourself.

bhk Over a year ago

Yeah I think I figured it out. I made it work anyways! :) Thanks!

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