Serialise FORM bodies with pydantic via type annotiations

Description

I started to use FastAPI and enjoyed serialisation of JSON bodies into pydantic models via type annotations and then I passed the form to my request handler and was surprised with AttributeError.

So I re-read the docs about forms and found its behaviour inconsistent with JSON bodies even though probably all code might already support this.

Example

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

app = FastAPI()


class Item(BaseModel):
    name: str
    another: str


@app.post("/item_form", response_model=Item)
async def find_item(r: Request):
    # current workaround
    form = await r.form()
    item = Item(**form)
    return item


@app.post("/item", response_model=Item)
async def find_item(r: Request, item: Item):
    # JSON Body works nicely
    print(item.name)
    return item


@app.post("/item_form_a", response_model=Item)
async def find_item(item: Item = Form(...)):
    # alternative 1
    # current: 422 Unprocessable Entity
    return item


class ItemForm(BaseModel):
    name: str = Form(...)
    another: str = Form(...)


@app.post("/item_form_b", response_model=ItemForm)
async def find_item(item: ItemForm):
    # alternative 2
    # current: AttributeError and 422 Unprocessable Entity
    print(item.name)
    return item

Motivation

I’m building a slack bot application which receives slash commands from Slack. POST request received from slack is an URL encoded body. https://api.slack.com/web#slack-web-api__basics__post-bodies__url-encoded-bodies

[Mause]

Sorry, should have given a full example:

from fastapi.testclient import TestClient
from fastapi import FastAPI, Depends, Form
from pydantic import BaseModel


app = FastAPI()


def form_body(cls):
    cls.__signature__ = cls.__signature__.replace(
        parameters=[
            arg.replace(default=Form(...))
            for arg in cls.__signature__.parameters.values()
        ]
    )
    return cls


@form_body
class Item(BaseModel):
    name: str
    another: str


@app.post('/test', response_model=Item)
def endpoint(item: Item = Depends(Item)):
    return item


tc = TestClient(app)


r = tc.post('/test', data={'name': 'name', 'another': 'another'})

assert r.status_code == 200
assert r.json() == {'name': 'name', 'another': 'another'}

[ArcLightSlavik]

@1oglop1 Could you close the issue