16. HTML Forms

from fastapi import FastAPI, Request, Form
from fastapi.templating import Jinja2Templates
from fastapi.responses import HTMLResponse, RedirectResponse

app = FastAPI()
templates = Jinja2Templates(directory="templates")

@app.get("/login", response_class=HTMLResponse)
def login_form(request: Request):
    return templates.TemplateResponse("login.html", {"request": request})

@app.post("/login")
def login(request: Request, username: str = Form(...), password: str = Form(...)):
    # Validate credentials
    if username == "admin" and password == "secret":
        return RedirectResponse(url="/dashboard", status_code=302)
    return templates.TemplateResponse(
        "login.html",
        {"request": request, "error": "Invalid credentials"}
    )

HTML Form Template

<form method="post" action="/login">
  <input type="text" name="username" placeholder="Username">
  <input type="password" name="password" placeholder="Password">
  <button type="submit">Login</button>
</form>
{% if error %}
  <p style="color:red">{{ error }}</p>
{% endif %}