Serialize a json string into a Pydantic model in a multipart Form

[Ran4]

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Example

I'm trying to serialize a json string sent in as a multipart form.

This is a minimally reproducible server:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Spec(BaseModel):
    some_field: str

@app.post("/foo")
async def foo(
        spec: Spec = Form(..., ),
        file: List[UploadFile] = File(...),
        ):
    return "success!"

The server will, when called with this request, as sent from python's requests library:

requests.post(url, data={"spec": spec.json()}, files={"file": (filename, file_handle)})

where spec is an instance of Spec above, return 422 with body

{"detail":[{"loc":["body","spec"],"msg":"value is not a valid dict","type":"type_error.dict"}]}

Environment

• OS: Ubuntu 20.04
• FastAPI Version 0.61.1
• Python version: 3.8.5

[Kludex]

This can help you: #2387 (comment)

[falkben]

I feel like I can understand what's going on with the following a bit better, but I believe they are essentially doing the same thing:

def as_form(cls):
    """ decorator for you pydantic model to turn them into form models """
    cls.__signature__ = cls.__signature__.replace(
        parameters=[
            arg.replace(default=Form(...) if arg.default is arg.empty else Form(arg.default))
            for arg in cls.__signature__.parameters.values()
        ]
    )
    return cls

then you use it like so:

class UserModel(BaseModel):
    username: str

@service.as_form
class UserModelForm(UserModel):
    """ form model """

any path operators that use UserModelForm accept form data. any path operators that use UserModel accept json

like so (for form data):

@router.post("/post_form")
def post_form(self, user: UserModel = Depends(UserModelForm)):
    return user

[Maksych]

@Ran4
I think this solution is what you need.

import fastapi
import pydantic

app = fastapi.FastAPI()


class User(pydantic.BaseModel):
    username: str


@app.post("/create-user-json")
def create_user_json(
    user: pydantic.Json[User] = fastapi.Form(...),
    file: fastapi.UploadFile = fastapi.File(...),
) -> None:
    ...

[dotX12]

@Ran4

https://github.com/dotX12/pyfa-converter

from datetime import datetime
from typing import Optional

from fastapi import FastAPI, UploadFile, File, Form
from pydantic import BaseModel, Field

from pyfa_converter import FormDepends, PyFaDepends

app = FastAPI()


class PostContractBodySchema(BaseModel):
    title: str = Field(..., description="Description title")
    date: Optional[datetime] = Field(
        None, description="Example: 2021-12-14T09:56:31.056Z"
    )


@app.post("/form-data-body")
async def example_foo_body_handler(
    data: PostContractBodySchema = FormDepends(PostContractBodySchema),
    # data1: PostContractBodySchema = PyFaDepends( # OR
    #         model= PostContractBodySchema, _type=Form
    #     ),
    document: UploadFile = File(...),
):
    return {"title": data.title, "date": data.date, "file_name": document.filename}