Deserialize JSON string to a class instance automatically in Python

Question

Assume I have several classes:

class Assignee:
    gid: str
    name: str
    
class Task:
    gid: str
    name: str
    created_at: datetime
    assignee: Assignee

and I receive a JSON, that I want to translate into my Task class instance:

{
 "gid": "#1",
 "name": "my task",
 "created_at": "2022-11-02T10:25:49.806Z",
 "assignee": {
  "gid": "#a1",
  "name": "assignee name"
 }
}

I need to get the strong typed result, not a dict. Is it possible to convert from JSON string to a class instance automatically? In C# we can do it with JsonConvert.DeserializeObject<Task>(json_string). In Python I found a jsonpickle library, but as far as I see, it cannot do it automatically. Am I missing anything?

In real life I have many more classes and JSON properties.

No... not automatically. I mean I am sure someone has written a library that does it automatically but nothing that is built into python — Alexander
– Alexander, Commented Dec 17, 2022 at 19:34
It's easy to learn the fields of a dataclass: Assignee.__dict__['__dataclass_fields__'].keys(). From there, you can probe what JSON .loads() gave you, and populate a newly created object. If you put the values into dict d, then Assignee(**d) produces what you want. Once you've put together an implementation you're happy with, please share it with us here. — J_H
– J_H, Commented Dec 17, 2022 at 19:44
It's not an automatic solution, I am looking for a library that can do the staff in one line, resolving data types. — Alex Sham
– Alex Sham, Commented Dec 17, 2022 at 20:07

Plagon · Accepted Answer · 2022-12-17 20:12:28Z

3

Use pydantic. It has type validation (and other cool features) and is very easy to use:

from pydantic import BaseModel
from datetime import datetime

class Assignee(BaseModel):
    gid: str
    name: str
    

class Task(BaseModel):
    gid: str
    name: str
    created_at: datetime
    assignee: Assignee


data = {
    "gid": "#1",
    "name": "my task",
    "created_at": "2022-11-02T10:25:49.806Z",
    "assignee": {
        "gid": "#a1",
        "name": "assignee name"
    }
}

Task(**data)
>>>Task(gid='#1', name='my task', created_at=datetime.datetime(2022, 11, 2, 10, 25, 49, 806000, tzinfo=datetime.timezone.utc), assignee=Assignee(gid='#a1', name='assignee name'))

answered Dec 17, 2022 at 20:12

Plagon

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Comments

chepner · Accepted Answer · 2022-12-17 19:34:49Z

0

Not quite what you are asking, but the json module already produces suitable dicts. All you need to do is define how to instantiate your classes given a dict.

Part of this requires you to assume what type each key's value must be instantiated as.

class Assignee:
    @classmethod
    def from_dict(cls, d):
        return cls(d['gid'], d['name'])

class Task:
    @classmethod
    def from_dict(cls, d):
        return cls(d['gid'], d['name'], d['created_at'], Assignee.from_dict(d['assignee']))

t = Task.from_dict(json.load('{"gid": "#1", ...}'))

answered Dec 17, 2022 at 19:34

chepner

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Comments

DJSDev · Accepted Answer · 2022-12-17 20:07:20Z

Assuming the json properties match the property names exactly in the classes, you can use kwargs in the constructors to put the object together.

import json

class Assignee:
    def __init__(self, gid, name):
        self.gid = gid
        self.name = name

class Task:
    def __init__(self, gid, name, created_at, assignee):
        self.gid = gid
        self.name = name
        self.created_at = created_at
        self.assignee = Assignee(**assignee)

json_string = '{ "gid": "123", "name": "Bob", "created_at": "1/1/1990", "assignee": { "gid": "234", "name": "Alice" } }'

# Instantiate Task with kwargs loaded from json
my_task = Task(**json.loads(json_string))

print(my_task.assignee.name)
# prints 'Alice'

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