Using pydantic Json Type as Form data type doesn't work

[Kludex]

Discussed in #9305

^{Originally posted by harpaj March 23, 2023}

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
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Example Code

from typing import Annotated

from fastapi import FastAPI, Form
from pydantic import Json, BaseModel

app = FastAPI()


class JsonListModel(BaseModel):
    json_list: Json[list[str]]


@app.post("/working")
async def working(json_list: Annotated[str, Form()]) -> list[str]:
    model = JsonListModel(json_list=json_list)
    return model.json_list


@app.post("/broken")
async def broken(json_list: Annotated[Json[list[str]], Form()] ) -> list[str]:
    return json_list

Description

In the example code above, I would expect working and broken to be approximately equivalent.
However, while working returns the parsed json_list as expected, broken fails with

{"detail":[{"loc":["body","json_list"],"msg":"JSON object must be str, bytes or bytearray","type":"type_error.json"}]}

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.95.0

Python Version

Python 3.10.8

Additional Context

sample request that fails with broken and works with working:

import requests
import json

headers = {"Content-Type": "application/x-www-form-urlencoded"}
response = requests.post(
    "http://0.0.0.0:8000/broken",
    data={"json_list": json.dumps(["abc", "def"])},
    headers=headers,
)
```</div>

[dayeondev]

Hi @Kludex @harpaj
As I've been reviewing this issue, a few questions have arisen.

1. Is the primary goal of this issue to enable the Form() function to directly receive data in JSON format? Or, is the focus more on providing documentation and examples for handling this indirectly?
2. Is there a need for detailed documentation on this phenomenon and its causes? Is the goal to provide guidelines to help users understand and appropriately manage this issue?