How do I successfully validate and parse htmx form data with a python/fastapi backend using BaseModel?

Question

I am having a difficult time successfully parsing and validating HTMX form data in a FastAPI backend.

When I send a POST request to my backend the associated data is stored in the body of the request and formatted as:

key=value&key_2=value_2

This is expected considering the default Content Type header of the request is application/x-www-form-urlencoded.

How am I meant to parse this data in FastAPI? The documentation says to install python-multipart (which I have done) and type the data as:

async def index_post (request: Request, data: Annotated[{type}, Form()]):

which works but seems excessive.

Is this the only way to access the data? Can I not leverage pydantic BaseModel classes here?

I would think something like

class FormData(BaseModel):
    key: str
    key_2: str
    key_3 str

@router.post("/post")
async def index_post(request: Request, data: FormData): (or) async def index_post(request: Request, data: Annotated[FormData, Form()]):
    print(data.key)
    print(data.key_2)
    print(data.key_3)
    return templates.TemplateResponse("main.html", {'request': request})

would work but I can't seem to find anything similar?

Is there a way to define a data structure with BaseModel here? And is using

async def index_post (request: Request, data: Annotated[{type}, Form()]):

the only way to extract the incoming form data?

---------- MRE INFO ----------

this successfully prints the value to the server console.

{% extends "/shared/_base.html" %}

{% block content %}
<div id="buttonParent">
    <form>
    <input type="text" name="testing" value="hello world">
    <button hx-post="/post"
            hx-trigger="click"
            hx-target="#buttonParent"
    >
        Click Me!
    </button>
    </form>
</div>
{% endblock %}

@router.post("/post")
async def index_post(request: Request, testing: Annotated[str, Form()]):
    print(testing)
    return templates.TemplateResponse("main.html", {'request': request})

Answer 1

Htmx doesn't do any additional modification of the FormData, so any library that parses regular HTML FormData will work with htmx. For FastAPI in particular, this answer has a section on FormData, which should work unmodified.