Is it possible to get pydantic v2 to dump json with sorted keys?

Question

In the pydantic v1 there was an option to add kwargs which would get passed to json.dumps via **dumps_kwargs. However, in pydantic v2 if you try to add extra kwargs to BaseModel.json() it fails with the error TypeError: `dumps_kwargs` keyword arguments are no longer supported.

Here is example code with a workaround using dict()/model_dump(). This is good enough as long as the types are simple, but it won't work for the more complex data types that pydantic knows how to serialize.

Is there a way to get sort_keys to work in pydantic v2 in general?

import json
from pydantic import BaseModel


class JsonTest(BaseModel):
    b_field: int
    a_field: str

obj = JsonTest(b_field=1, a_field="one")

# this worked in pydantic v1 but raises a TypeError in v2
# print(obj.json(sort_keys=True)

print(obj.model_dump_json())
# {"b_field":1,"a_field":"one"}

# workaround for simple objects
print(json.dumps(obj.model_dump(), sort_keys=True))
# {"a_field": "one", "b_field": 1}

Answer 1

I'm not sure whether it is an elegant solution but you could leverage the fact that dictionaries (since python 3.7) preserve an order of elements:

from typing import Any, Dict
from pydantic import BaseModel, model_serializer


class JsonTest(BaseModel):
    b_field: int
    c_field: int
    a_field: str

    @model_serializer(when_used='json')
    def sort_model(self) -> Dict[str, Any]:
        return dict(sorted(self.model_dump().items()))


obj = JsonTest(b_field=1, a_field="one", c_field=0)
print(obj.model_dump_json())
# {"a_field":"one","b_field":1,"c_field":0}