FastHTML: HTML applications, the Python way

Many Python developers prefer to work in their favourite language rather than grapple with the complexities of HTML and other web technologies; this can make web application development more time-consuming, less efficient and less enjoyable.
FastHTML takes a different approach by allowing you to use Python exclusively to create web applications, combining this with a minimalist philosophy inspired by FastAPI.
Thanks to the use of tried-and-tested technologies such as Starlette and HTMX, it focuses on the developer experience to create web applications with minimal effort.
I will guide you through this framework, exploring its possibilities and sharing my journey of discovery.

Transcript

1. FastHTML HTML applications the Python way Iacopo Spalletti @ Nephila PyCon Italia 2026 Iacopo Spalletti - @yakkys
2. Iacopo Spalletti - @yakkys
3. the project internal tool token storage multiple projects once upon a time … Iacopo Spalletti - @yakkys
4. it worked MiniDataAPI external auth happy a story Iacopo Spalletti - @yakkys
5. then it didn’t complex data model permission system MiniDataAPI → SQLAlchemy ? a story Iacopo Spalletti - @yakkys
6. Django enters the picture a story Iacopo Spalletti - @yakkys
7. Django enters the picture No regrets. a story Iacopo Spalletti - @yakkys
8. the problem your Python is elegant your frontend is someone else’s problem Iacopo Spalletti - @yakkys
9. backend Django Flask FastAPI the stack Iacopo Spalletti - @yakkys
10. frontend Jinja / Django template DRF / FastAPI React / Vue npm vite … the stack Iacopo Spalletti - @yakkys
11. frontend the stack Iacopo Spalletti - @yakkys
12. the result two languages two mental models two ecosystems two dependency systems … one application. the stack Iacopo Spalletti - @yakkys
13. the pattern backend returns JSON JavaScript assembles the UI A hard to maintain two-way contract the dominant approach Iacopo Spalletti - @yakkys
14. the question is this always necessary? the dominant approach Iacopo Spalletti - @yakkys
15. the original model 1991 HTTP + HTML server returns HTML → browser renders it the web was right all along Iacopo Spalletti - @yakkys
16. the insight nothing was broken More interactions everywhere Javascript is the only way to go No longer the case the web was right all along Iacopo Spalletti - @yakkys
17. hypermedia API-based backend → JSON → JavaScript → UI hypermedia-based backend → HTML → browser Iacopo Spalletti - @yakkys
18. hypermedia Client side state management ⨯ Server as the source of truth ✓ Iacopo Spalletti - @yakkys
19. HTMX the missing piece any element · any event any event · any HTTP method partial DOM updates 14KB · no build step · no framework Iacopo Spalletti - @yakkys
20. FastHTML built on Starlette · HTMX inspired by FastAPI · for hypermedia async-native < 1000 lines of library code Iacopo Spalletti - @yakkys
21. the mental model Q: how do I manage client state? Iacopo Spalletti - @yakkys
22. the mental model Q: how do I manage client state? Q: what HTML the server should return? Iacopo Spalletti - @yakkys
23. the mental model server as source of truth Iacopo Spalletti - @yakkys
24. the model Python objects HTML output 1 Div(P("Hello")) 1 <div><p>Hello</p></div> Iacopo Spalletti - @yakkys
25. the model routes return UI no templates no context switching Iacopo Spalletti - @yakkys
26. get started 1 pip install python-fasthtml Iacopo Spalletti - @yakkys
27. get started docs https://fastht.ml/docs/ https://hypermedia.systems/ https://github.com/yakky/FastHTML-example Iacopo Spalletti - @yakkys
28. get started Iacopo Spalletti - @yakkys
29. minimal app 18 serve(port=5001) 1 from fasthtml.common import ( 2 H1, Button, Div, Form, 3 Link, Style, Textarea, 4 fast_app, serve, 5 ) 6 7 app, rt = fast_app( 8 hdrs=[ 9 Link(rel="stylesheet", href=PICO_CDN), 10 Style(OVERRIDES), 11 ] 12 ) 13 14 @rt("/") 15 def get(): 16 return _page(history) 17 Iacopo Spalletti - @yakkys
30. components Python functions are your components minimal app 13 def _page(msgs: list) -> tuple: 14 return ( 15 Title("Claude Chat"), 16 Div(H1("Claude Chat"), _messages_area(msgs), _input_bar()), 17 Script(JS) 18 ) 1 def _render_message(msg: dict) -> Div: 2 role = "You" if msg["role"] == "user" else "Claude" 3 return Div( 4 Div(role, cls="msg-role"), 5 Div(msg["content"], cls="msg-bubble"), 6 cls=f"msg {msg['role']}" 7 ) 8 9 def _messages_area(msgs: list) -> Div: 10 inner = [_render_message(m) for m in msgs] 11 return Div(*inner, id="messages") 12 Iacopo Spalletti - @yakkys
31. Component library is still young https://github.com/AnswerDotAI/fasthtml-example https://github.com/amosgyamfi/awesome-fasthtml components Iacopo Spalletti - @yakkys
32. Bonus: Generator https://h2f.answer.ai/ HTML -> FastHTML components Iacopo Spalletti - @yakkys
33. interactivity click → request → HTML fragment → DOM update minimal app 17 @rt("/clear", methods=["POST"]) 18 def post_clear(): 19 history.clear() 20 return _page(history) 1 def _input_bar() -> Div: 2 return Div( 3 Form( 4 Textarea(id="user-input", rows="1"), 5 Button("Send", type="submit"), 6 id="chat-form", 7 ), 8 Button( 9 "Clear conversation", 10 hx_post="/clear", 11 hx_target="body", 12 hx_swap="outerHTML", 13 ), 14 id="input-bar", 15 ) 16 Iacopo Spalletti - @yakkys
34. what comes in the box sessions · auth Databases via MiniDataAPI PicoCSS WebSockets · SSE Iacopo Spalletti - @yakkys
35. MiniDataAPI what comes in the box 1 app, rt, todos, Todo = fast_app( 2 'data/todos.db', 3 id=int, task=str, done=bool, pk='id' 4 ) Iacopo Spalletti - @yakkys
36. MiniDataAPI no migration no complete ORM what comes in the box Iacopo Spalletti - @yakkys
37. querying MiniDataAPI 8 todo = todos.get(id) 9 return Article(H2(todo.task)) 1 @rt("/") 2 def get(): 3 items = todos() 4 return Ul(*[Li(t.task) for t in items]) 5 6 @rt("/todos/{id}") 7 def get_detail(id: int): Iacopo Spalletti - @yakkys
38. searching MiniDataAPI 7 todos(where="done=0", order_by='task', limit=5) # combined 1 todos(where="done=0") # filtered 2 3 todos(order_by='created_at') # sorted 4 5 todos(limit=10, offset=20) # paginated 6 Iacopo Spalletti - @yakkys
39. safe filtering MiniDataAPI 8 @rt("/search") 9 def get(q: str): 10 results = todos("task like ?", (f"%{q}%",)) 11 return Ul(*[Li(t.task) for t in results]) 1 # never interpolate user input directly 2 todos(where=f"task='{q}'") # ⚠ SQL injection 3 4 # use positional placeholders instead 5 todos("task=?", (q,)) 6 7 # in a route Iacopo Spalletti - @yakkys
40. [] — access by primary key MiniDataAPI 11 try: 12 todos[999] 13 except NotFoundError: 14 ... 1 # integer key 2 todo = todos[1] 3 4 # string key 5 user = users['Alma'] 6 7 # compound key 8 pub = publications['Alma', 2019] 9 10 # missing key raises, not returns None Iacopo Spalletti - @yakkys
41. .xtra() MiniDataAPI 7 # writes enforce the constraint too 8 todos.insert(Todo(task="learn FastHTML")) 9 # → row has user_id set automatically 1 # lock the table to the current user's records 2 todos.xtra(user_id=session['user_id']) 3 4 # all subsequent reads are automatically filtered 5 todos() # only this user's todos 6 Iacopo Spalletti - @yakkys
42. in — membership testing MiniDataAPI 9 # respects .xtra() scope 10 todos.xtra(user_id=42) 11 99 in todos # False even if todo 99 exists for another user 1 # simple key 2 'Alma' in users # True 3 99 in todos # False 4 5 # compound key 6 ['Alma', 2019] in publications # True 7 ('John', 1967) in publications # False 8 Iacopo Spalletti - @yakkys
43. CRUD MiniDataAPI 9 @rt("/todos/{id}") 10 def delete(id: int): 11 todos.delete(id) 1 @rt("/") 2 def post(todo: Todo): 3 return todos.insert(todo), mk_input(hx_swap_oob='true') 4 5 @rt("/todos/{id}") 6 def put(todo: Todo): 7 return todos.upsert(todo) 8 Iacopo Spalletti - @yakkys
44. database backends MiniDataAPI 5 # fastsql — any SQLAlchemy database 6 from fastsql import * 7 db = Database('postgresql://...') 1 # fastlite — SQLite (default) 2 from fastlite import * 3 db = database('app.db') 4 8 9 # the rest of your code stays identical Iacopo Spalletti - @yakkys
45. build your own library components ui.py forms.py layout.py components Iacopo Spalletti - @yakkys
46. build your own library components 1 # components/ui.py 2 from fasthtml.common import * 3 4 def Card(title: str, *content, cls: str = "") -> Article: 5 return Article( 6 Header(H3(title)), 7 *content, 8 cls=f"card {cls}".strip() 9 ) 10 11 def Badge(text: str, variant: str = "primary") -> Span: 12 return Span(text, cls=f"badge badge-{variant}") Iacopo Spalletti - @yakkys
47. composition components 10 def TodoList(items: list[Todo]) -> Ul: 11 return Ul(*[TodoItem(t) for t in items], id="todo-list") 1 def TodoItem(todo: Todo) -> Li: 2 return Li( 3 Span(todo.task), 4 Badge("done" if todo.done else "pending", 5 variant="success" if todo.done else "secondary"), 6 Button("✓ ", hx_patch=f"/todos/{todo.id}", 7 hx_target="closest li", hx_swap="outerHTML"), 8 ) 9 Iacopo Spalletti - @yakkys
48. composition pure Python testable with pytest no browser needed components Iacopo Spalletti - @yakkys
49. background tasks 5 async def post(email: str): 7 return P("Export started — check your inbox"), task 1 def send_report(email: str): 2 mailer.send(email, subject="Report ready") 3 4 @rt("/export") 6 task = BackgroundTask(send_report, email=email) Iacopo Spalletti - @yakkys
50. SSE streaming demo 14 15 @rt("/chat") 16 def get_chat(request: Request) -> StreamingResponse: 17 history.append({"role": "user", "content": request.query_params["message"]}) 18 return StreamingResponse( 19 _stream_claude(DEFAULT_MODEL, DEFAULT_SYSTEM, history), 20 media_type="text/event-stream" 21 ) 1 def _sse_frame(text: str) -> bytes: 2 frame = "".join(f"data: {line}\n" for line in text.split("\n")) 3 return (frame + "\n").encode() 4 5 def _stream_claude(model, system, messages) -> Iterator[bytes]: 6 client = anthropic.Anthropic() 7 with client.messages.stream( 8 model=model, max_tokens=MAX_TOKENS, 9 system=system, messages=messages, 10 ) as stream: 11 for text in stream.text_stream: 12 yield _sse_frame(text) 13 yield b"data: [DONE]\n\n" Iacopo Spalletti - @yakkys
51. the use cases internal tools · dashboards · prototypes single developer · small team fluent with FastAPI where it shines Iacopo Spalletti - @yakkys
52. the gap between Streamlit and Django where it shines Iacopo Spalletti - @yakkys
53. missing pieces no admin no ORM · no migrations no asset fingerprinting · no cache busting the limits Iacopo Spalletti - @yakkys
54. ecosystem small community · young project fewer plugins · fewer answers on Stack Overflow the limits Iacopo Spalletti - @yakkys
55. But Easily fit in AI context Less hallucinations Easy to AI-code a full FastHTML app ecosystem Iacopo Spalletti - @yakkys
56. our story, revisited simple model → FastHTML ✓ complex model + permissions → FastHTML ✗ know your data model before you choose your framework Iacopo Spalletti - @yakkys
57. the readability question vs Is this a concern? Readability counts Although practicality beats purity 1 Div(P("hello")) 1 <div><p>hello</p></div> Iacopo Spalletti - @yakkys
58. Iacopo Spalletti Founder and CTO @Nephila_digital Djangonaut and open source developer @yakky@mastodon.social https://github.com/yakky https://www.linkedin.com/in/iacopospalletti/ https://speakerdeck.com/yakky Iacopo Spalletti - @yakkys
59. Questions? Iacopo Spalletti - @yakkys
60. Thanks! Iacopo Spalletti - @yakkys
61. Iacopo Spalletti - @yakkys