FastAPI don't call pydantic to json

[samsja]

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• [X] I added a very descriptive title to this issue.
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• [X] I already searched in Google "How to X in FastAPI" and didn't find any information.
• [X] I already read and followed all the tutorial in the docs and didn't find an answer.
• [X] I already checked if it is not related to FastAPI but to Pydantic.
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Example Code

import json

from fastapi import FastAPI
from pydantic import BaseModel


def json_dump(*args, **kwargs):
    print('HELLO')
    return json.dumps(*args, **kwargs)

class MyModel(BaseModel):

    class Config:
        json_dumps = json_dump

    a : int


data = MyModel(a=1)

data.json()

app = FastAPI()

@app.post("/hello/", response_model=MyModel)
async def hello() -> MyModel:
    return MyModel(a=1)


import uvicorn

uvicorn.run(app)

### on client.py side
from httpx import Client

with Client(base_url="http://localhost:8000") as ac:
    resp = ac.post(
        "/hello/",
    )

>>> HELLO

Description

This shows the HELLO is called only once. Which means FastAPI is not using the pydantic to json function.

By looking at the code we can see that there is a custom function written on FastAPI side. Is there a way to use the json method from pydantic instead ?

Thanks in advance

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.87.0

Python Version

3.9.10

Additional Context

No response

[odiseo0]

I think you could use def dict in Pydantic. FastAPI uses jsonable_encoder to create a dict from a Pydantic model then serializes the result with json, orjson, ujson etc

[yinziyan1206]

you can set json_encoders in pydantic model instead of json_dumps for example: json_encoders = {float: lambda x: float(x)}

[samsja]

Yes indeed json_encoders is a way to do it. Unfortunately it will be called not directly on the BaseModel but at its child levels. In the code it first goes into the BaseModel collect the json encoder and then call it but only on the child.

My question is why not using directly BaseModel.json() in FastAPI ? There might be a reason that I am not aware like something related to SQLAlchemy. But from what I saw it looks like a redundancy with pydantic code

[samsja]

let

I think you could use def dict in Pydantic. FastAPI uses jsonable_encoder to create a dict from a Pydantic model then serializes the result with json, orjson, ujson etc

Let me check if I can solve my problem with this. Thanks for the feedback

[samsja]

I found out a solution with json_encoder but I still wondering why not just use BaseModel.json() from pydantic