How to parse a JSON object into a python dataclass without third party library?

Question

I want to parse json and save it in dataclasses to emulate DTO. Currently, I ahve to manually pass all the json fields to dataclass. I wanted to know is there a way I can do it by just adding the json parsed dict ie. "dejlog" to dataclass and all the fields are populated automactically.

from dataclasses import dataclass,  asdict


@dataclass
class Dejlog(Dataclass):
    PK: str
    SK: str
    eventtype: str
    result: str
    type: str
    status: str

def lambda_handler(event, context):
    try:
        dejlog = json.loads(event['body'])

        x = Dejlog(dejlog['PK'])

        print(x)

        print(x.PK)

load json to dict and then intérnable unpack into le dataclass instance — Wizard.Ritvik
– Wizard.Ritvik, Commented Apr 26, 2022 at 11:52
If dejlog contains the expected keys, what you want is x = Dejlog(**dejlog). — Serge Ballesta
– Serge Ballesta, Commented Apr 26, 2022 at 11:59
@SergeBallesta Yes thank you but If there is a new (unexpected ) key added to json then unpacking will fail. Any way to overcome it. Also if some fields in json contain list or object, and in data class I have mentioned str. How to handle it. — sji gshan
– sji gshan, Commented Apr 26, 2022 at 13:18

Don · Accepted Answer · 2022-04-26 12:14:23Z

19

As mentioned in other comments you can use the in-built json lib as so:

from dataclasses import dataclass
import json

json_data_str = """
{
   "PK" : "Foo",
   "SK" : "Bar",
   "eventtype" : "blah",
   "result" : "something",
   "type" : "badger",
   "status" : "active"
}
"""

@dataclass
class Dejlog:
    PK: str
    SK: str
    eventtype: str
    result: str
    type: str
    status: str


json_obj = json.loads(json_data_str)
dejlogInstance = Dejlog(**json_obj)

print(dejlogInstance)

answered Apr 26, 2022 at 12:14

Don

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1 Comment

danvk Over a year ago

Note that this is shallow: it won't work if your dataclass contains other dataclasses. For that, see stackoverflow.com/q/69059290/388951.

Chris Burgoyne · Accepted Answer · 2023-04-25 16:25:17Z

Unpacking works as long as you don't have any unexpected keys in the json object, otherwise you'll get a TypeError. An alternative is to use a classmethod to create instances of the dataclass. Building on the earlier example:

json_data_str = """
{
   "PK" : "Foo",
   "SK" : "Bar",
   "eventtype" : "blah",
   "result" : "something",
   "type" : "badger",
   "status" : "active",
   "unexpected": "I did not expect this"
}
"""

@dataclass
class Dejlog:
    PK: str
    SK: str
    eventtype: str
    result: str
    type: str
    status: str
    
    @classmethod
    def from_dict(cls, data):
        return cls(
            PK = data.get('PK'),
            SK = data.get('SK'),
            eventtype = data.get('eventtype'),
            result=data.get('result'),
            type=data.get('type'),
            status=data.get('status')
        )

json_obj = json.loads(json_data_str)
dejlogInstance = Dejlog.from_dict(json_obj)

John Hillery · Accepted Answer · 2025-01-22 19:32:53Z

1

Here is a simple classmethod that handles missing or extra keys. Building on an earlier answer:

import dataclasses
import json

json_data_str = """
{
   "PK" : "Foo",
   "SK" : "Bar",
   "eventtype" : "blah",
   "result" : "something",
   "type" : "badger",
   "status" : "active",
   "unexpected": "I did not expect this"
}
"""

@dataclasses.dataclass
class Dejlog:
    PK: str
    SK: str
    eventtype: str
    result: str
    type: str
    status: str

    @classmethod
    def from_dict(cls, data):
        return cls(
            *[data.get(fld.name)
              for fld in dataclasses.fields(Dejlog)]
        )

json_obj = json.loads(json_data_str)
dejlogInstance = Dejlog.from_dict(json_obj)

answered Jan 22 at 19:32

John Hillery

112 bronze badges

Comments

Yechiel babani · Accepted Answer · 2024-12-18 19:48:46Z

0

You can load json to your dataclass object using hook_object param like this:

import json
from dataclasses import dataclass


@dataclass
class Dejlog:
    PK: str
    SK: str
    eventtype: str
    result: str
    type: str
    status: str


my_obj = json.loads(
    open('/path/to/file', 'r').read(),
    object_hook=lambda args: Dejlog(**args)
)

answered Dec 18, 2024 at 19:48

Yechiel babani

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