💭 python - How to use a Pydantic model with Form data in FastAPI...
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!https://stackoverflow.com/questions/60127234/how-to-use-a-pydantic-model-with-form-data-in-fastapi

Date: August 8, 2023

How to use a Pydantic model with Form data in FastAPI?
 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
 Stack Overflow · stackoverflow.com [1]

I went down the route of leveraging the `json-enc` extention in htmx, but later realized that this completely breaks browsers/users who do not wish to use javascript. While most of the web would feel quite broken with javascript disabled, I don't want to contribute to that without good reason.

Taking a second look into this issue, rather than using `json-enc`, and using as_form to get form data into a model keeps the nice DX fo everything being a pydantic model, but the site still works without js. with js htmx kicks in, you get a spa like experience by loading partials onto the page, and without, you just get a full page reload.

## the implementation

copied from https://stackoverflow.com/questions/60127234/how-to-use-a-pydantic-model-with-form-data-in-fastapi

``` python
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

``` python
@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
```

!!! note

 This post is a thought [2]. It's a short note that I make
 about someone else's content online #thoughts [3]

References:
[1]: https://stackoverflow.com/questions/60127234/how-to-use-a-pydantic-model-with-form-data-in-fastapi
[2]: /thoughts/
[3]: /tags/thoughts/