How to use a Pydantic model with Form data in FastAPI?

Question

I am trying to submit data from HTML forms and validate it with a Pydantic model.

Using this code

from fastapi import FastAPI, Form
from pydantic import BaseModel
from starlette.responses import HTMLResponse


app = FastAPI()

@app.get("/form", response_class=HTMLResponse)
def form_get():
    return '''<form method="post"> 
    <input type="text" name="no" value="1"/> 
    <input type="text" name="nm" value="abcd"/> 
    <input type="submit"/> 
    </form>'''


class SimpleModel(BaseModel):
    no: int
    nm: str = ""

@app.post("/form", response_model=SimpleModel)
def form_post(form_data: SimpleModel = Form(...)):
    return form_data

However, I get the HTTP error: "422 Unprocessable Entity"

{
    "detail": [
        {
            "loc": [
                "body",
                "form_data"
            ],
            "msg": "field required",
            "type": "value_error.missing"
        }
    ]
}

The equivalent curl command (generated by Firefox) is

curl 'http://localhost:8001/form' -H 'Content-Type: application/x-www-form-urlencoded' --data 'no=1&nm=abcd'

Here the request body contains no=1&nm=abcd.

What am I doing wrong?

Answer 1

I found a solution that can help us to use Pydantic with FastAPI forms :)

My code:

class AnyForm(BaseModel):
    any_param: str
    any_other_param: int = 1

    @classmethod
    def as_form(
        cls,
        any_param: str = Form(...),
        any_other_param: int = Form(1)
    ) -> AnyForm:
        return cls(any_param=any_param, any_other_param=any_other_param)

@router.post('')
async def any_view(form_data: AnyForm = Depends(AnyForm.as_form)):
        ...

It's shown in the Swagger as a usual form.

It can be more generic as a decorator:

import inspect
from typing import Type

from fastapi import Form
from pydantic import BaseModel
from pydantic.fields import ModelField

def as_form(cls: Type[BaseModel]):
    new_parameters = []

    for field_name, model_field in cls.__fields__.items():
        model_field: ModelField  # type: ignore

        new_parameters.append(
             inspect.Parameter(
                 model_field.alias,
                 inspect.Parameter.POSITIONAL_ONLY,
                 default=Form(...) if model_field.required else Form(model_field.default),
                 annotation=model_field.outer_type_,
             )
         )

    async def as_form_func(**data):
        return cls(**data)

    sig = inspect.signature(as_form_func)
    sig = sig.replace(parameters=new_parameters)
    as_form_func.__signature__ = sig  # type: ignore
    setattr(cls, 'as_form', as_form_func)
    return cls

And the usage looks like

@as_form
class Test(BaseModel):
    param: str
    a: int = 1
    b: str = '2342'
    c: bool = False
    d: Optional[float] = None


@router.post('/me', response_model=Test)
async def me(request: Request, form: Test = Depends(Test.as_form)):
    return form

Answer 2

you can use data-form like below:

@app.post("/form", response_model=SimpleModel)
def form_post(no: int = Form(...),nm: str = Form(...)):
    return SimpleModel(no=no,nm=nm)

Answer 3

I implemented the solution found here Mause solution and it seemed to work

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'}

Answer 4

If you're only looking at abstracting the form data into a class you can do it with a plain class

from fastapi import Form, Depends

class AnyForm:
    def __init__(self, any_param: str = Form(...), any_other_param: int = Form(1)):
        self.any_param = any_param
        self.any_other_param = any_other_param

    def __str__(self):
        return "AnyForm " + str(self.__dict__)

@app.post('/me')
async def me(form: AnyForm = Depends()):
    print(form)
    return form

And it can also be turned into a Pydantic Model

from uuid import UUID, uuid4
from fastapi import Form, Depends
from pydantic import BaseModel

class AnyForm(BaseModel):
    id: UUID
    any_param: str
    any_other_param: int

    def __init__(self, any_param: str = Form(...), any_other_param: int = Form(1)):
        id = uuid4()
        super().__init__(id, any_param, any_other_param)

@app.post('/me')
async def me(form: AnyForm = Depends()):
    print(form)
    return form