Please use model_validate_json() for JSON in request body

[methane]

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Example Code

from fastapi import FastAPI, Body
from pydantic import BaseModel, ConfigDict
from enum import IntEnum


app = FastAPI()


class Color(IntEnum):
    blue = 1
    red = 2


class Item(BaseModel):
    name: str
    color: Color
    model_config = ConfigDict(strict=True)


@app.post("/item")
async def create_item(item: Item = Body()):
    print(repr(item))
    return {}

if __name__ == '__main__':
    print(repr(Item.model_validate_json('{"name":"cup", "color": 1}')))

Description

Pydantic v2 has model_validate_json, that parses JSON and validate it with JSON mode.
FastAPI 0.100.0 doesn't use it to parse JSON for now. So it is difficult to use strict mode for models.

uvicorn app:app --reload
...
> curl -X 'POST' \
        'http://127.0.0.1:8000/item' \
        -H 'accept: application/json' \
        -H 'Content-Type: application/json' \
        -d '{
    "name": "cup",
    "color": 1
  }' | jq .
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   232  100   199  100    33  27539   4566 --:--:-- --:--:-- --:--:--  226k
{
  "detail": [
    {
      "type": "is_instance_of",
      "loc": [
        "body",
        "color"
      ],
      "msg": "Input should be an instance of Color",
      "input": 1,
      "ctx": {
        "class": "Color"
      },
      "url": "https://errors.pydantic.dev/2.1.2/v/is_instance_of"
    }
  ]
}

Operating System

Linux, macOS

Operating System Details

No response

FastAPI Version

0.100.0

Python Version

3.11.3

Additional Context

No response

[ghost]

At the moment, the following implementation using field_validator with 'before' might be a workaround:

class Item(BaseModel):
    name: str
    color: Color
    model_config = ConfigDict(strict=True)

    @field_validator('color', mode='before')
    def value_to_enum(cls, v):
        if isinstance(v, int):
            return Color(v)
        return v

[PaulVanSchayck]

This seems still to be the case with FastAPI 0.103.1. The workaround resolves it, but is a bit ugly. Should we create an issue for this? I think it's a bug.

[TomerGodinger]

It is still present in FastAPI 0.111.1.

@tiangolo Can we please make this into an issue? It's problematic that you can't use enums with strict validation, and the workaround here needs to be added to every class that includes an enum (creating a general solution in a base class that covers all cases is... problematic, at best).