RailUI

Python-first, zero-runtime fullstack web framework.

Write reactive single-page applications entirely in Python. RailUI compiles your Python DSL directly into vanilla JavaScript/HTML/CSS bundles no Node.js, no JS framework, no runtime interpreter.

from railui.all import *

count, setCount = createSignal(0)

app = App("My App")

@app.route("/")
def home() -> Component:
    return Container(
        Text("Count: ", count()),
        Button("+1", on_click=setCount(count() + 1)),
        class_name="flex gap-4 p-6",
    )

if __name__ == "__main__":
    app.build()

---

Features

Pure Python DSL	Operator overloading compiles count() + 1 → sig_1() + 1 in JS — no Python runtime needed
Reactive Signals	First-class createSignal, createStore, useComputed, createEffect
Server Actions	@server_action decorator — call Python functions from the browser via auto-generated fetch()
File-Based Routing	Drop a .py file in pages/, get a route. Supports dynamic [id] segments
SPA Router	History API client-side navigation with scoped effects and lifecycle
Component System	15+ built-in components: Container, Button, Input, Show, Each, Form, Link, Image, Head, Suspense, slots, and more
Two-Way Binding	bind=signal on inputs — reactive form handling in one line
Animations	12 pre-built WAAPI animations: fade_in, slide_in_up, spin, bounce, shake, pulse + custom keyframes
Conditional Classes	class_list={"bg-red-500": has_error()}
Tailwind-like CSS	150+ utility classes compiled at build time — zero runtime CSS
JS Bridge	30+ utilities: log, alert, set_timeout, navigate, local_storage_set, copy_to_clipboard, on_mount, on_destroy
JS Namespace Wrappers	Math.floor(x), JSON.stringify(), Object.keys(), document.getElementById() — all compile to native JS
Named Slots	Slot/SlotFill for composable layouts
HMR	Dev server with file watcher and SSE-based hot reload
Zero Runtime	Output is pure vanilla JS (~1KB runtime) — no framework shipped to the browser
Deploy Anywhere	Railway (full-stack with server actions), Vercel (static SPA), or any static host

---

Installation

pip install railui

Requires Python ≥ 3.12.

---

Quick Start

1. Create a project

railui new my-app
cd my-app

Or manually create this structure:

my-app/
  main.py           # App entry point
  layout.py         # Shared layout (optional)
  pages/
    index.py        # Route: /
    counter.py      # Route: /counter
  public/           # Static assets (copied to build)
  railui.config.json

2. Define a page

# pages/counter.py
from railui.all import *

def page() -> Component:
    count, setCount = createSignal(0)
    return Container(
        Text("Count: ", count(), class_name="text-2xl font-bold"),
        Button("+1", on_click=setCount(count() + 1), class_name="px-4 py-2 bg-blue-600 text-white rounded"),
    )

3. Wire it up

# main.py
from railui.all import *

app = App("My Counter App")
app.discover_pages("pages")

if __name__ == "__main__":
    app.build()

4. Build

railui build
# Output: dist/index.html + app.{hash}.js + app.{hash}.css

5. Dev server

railui dev
# Opens http://127.0.0.1:5173 — HMR enabled

---

Core Concepts

The DSL — Python that becomes JavaScript

RailUI overloads Python operators so expressions like count() + 1 produce JavaScript AST nodes instead of executing immediately. This is how you write reactive logic in pure Python.

count, setCount = createSignal(10)

# These compile to JavaScript, they don't run in Python:
count() + 1         # → sig_1() + 1
count() * 2         # → sig_1() * 2
count() > 5         # → sig_1() > 5
count() == 0        # → sig_1() === 0 (=== in JS)
~show_details()     # → !sig_2()
count() + 1 * 2     # → (sig_1() + (1 * 2))

# Property access uses optional chaining:
user_data().name        # → sig_3()?.name
user_data().address.city # → sig_3()?.address?.city

# Index access:
items()[0]              # → sig_4()?.[0]

When you need raw JS, use RawJS:

RawJS("console.log('hello')")

Signals — Reactive State

# Simple signal
name, setName = createSignal("Alice")
age, setAge = createSignal(30)

# Reading (inside component tree — compiles to JS):
Text(name())           # reactive text node
Text(age() + 1)        # reactive expression

# Writing (inside event handlers — compiles to JS):
Button("Older", on_click=setAge(age() + 1))

# Computed value:
double_age = useComputed(lambda: age() * 2)
Text(double_age)       # reactive computed

# Side effect on every signal change:
createEffect(log("age changed to: ", age()))

# Store (grouped signals):
user = createStore({"name": "Alice", "role": "admin"})
Text(user.name())                    # reactive getter
Button("Reset", on_click=user.set_name(""))  # setter
Input(bind=user.name)                # two-way binding

Components

Container(
    Text("Hello", class_name="text-xl font-bold"),
    Button("Click", on_click=log("clicked"), class_name="px-4 py-2 bg-blue-500 text-white rounded"),
    class_name="flex flex-col gap-4 p-6",
    id="main-container",
)

---

Routing

File-Based Routing

File	Route
pages/index.py	/
pages/dashboard.py	/dashboard
pages/blog/index.py	/blog
pages/users/[id].py	/users/:id

Each page module must export a page() function returning a Component:

# pages/about.py
from railui.all import *

def page() -> Component:
    return Container(
        Text("About Us", class_name="text-3xl font-bold"),
    )

Programmatic Routing

app = App()

@app.route("/hello")
def hello_page() -> Component:
    return Text("Hello!")

@app.route("/user/<name>")
def user_page(name: str) -> Component:
    return Text(f"User: {name}")

404 Page

def not_found() -> Component:
    return Container(
        Text("404 — Page Not Found"),
        Link("Go home", href="/"),
    )

app.set_not_found(not_found)

---

State Management

createSignal

count, setCount = createSignal(0)
Text(count())                               # reactive display
Button("+", on_click=setCount(count() + 1))  # reactive update

createStore

user = createStore({
    "name": "Alice",
    "role": "admin",
    "is_logged_in": True,
})

Text(user.name())           # → sig_1()
Input(bind=user.name)       # two-way binding
Button("Logout", on_click=user.set_is_logged_in(False))

useFetch

posts, setPosts = createSignal([])
loading, setLoading = createSignal(True)

useFetch(
    url="https://api.example.com/posts",
    on_success=setPosts,
    loading=setLoading,
    on_error=log("Failed to load posts"),
)

Suspense(
    Each(items=posts, render_fn=lambda post, i: Text(post.title)),
    fallback=Text("Loading..."),
    loading=loading(),
)

createEffect

createEffect(log("count changed to: ", count()))

---

Event Handling

Button(
    "Click me",
    on_click=log("clicked!"),
    on_dblclick=setCount(count() + 2),
    on_mouseenter=log("hover!"),
)

Input(
    on_input=setName(event_value()),    # event.target.value
    on_change=log("committed: ", name()),
    on_focus=log("focused"),
    on_blur=log("blurred"),
)

Form(
    on_submit=runSequence(prevent_default(), log("submitted")),
)

Two-Way Binding

username, setUsername = createSignal("")

Input(
    bind=username,                    # two-way binding — one line
    placeholder="Enter username...",
    class_name="px-4 py-2 border rounded",
)

---

Event Utility Functions

prevent_default()      # → event.preventDefault()
stop_propagation()     # → event.stopPropagation()
event_value()          # → event.target.value

---

Conditional Rendering — Show

is_visible, setVisible = createSignal(False)

Show(
    Text("This is visible when the signal is truthy"),
    when=is_visible(),
    on_mount=log("BECAME VISIBLE"),
    on_update=log("UPDATED"),
    on_unmount=log("HIDDEN"),
)

# With fallback content:
Show(
    Text("Content loaded!"),
    when=is_loaded(),
    fallback=Text("Loading...", class_name="text-gray-500"),
)

---

List Rendering — Each

todos, setTodos = createSignal([
    {"title": "Learn RailUI"},
    {"title": "Build something awesome"},
])

Each(
    items=todos,
    render_fn=lambda item, index: Container(
        Text(index, class_name="text-gray-400 mr-2"),
        Text(item.title, class_name="font-semibold"),
        class_name="p-3 border-b",
    ),
    on_mount=log("List mounted!"),
    on_update=log("List updated!"),
)

# Immutable array operations:
Button("Add", on_click=setTodos(Array.append(todos(), {"title": "New task"})))
Button("Remove", on_click=setTodos(Array.remove(todos(), 0)))

---

Server Actions

Call Python functions from the browser — no API boilerplate.

Define

# pages/users.py
from railui.all import *

@server_action
def save_user(name: str):
    print(f"Saving {name} to database...")
    return {"status": "ok", "message": f"User {name} saved!"}

def page() -> Component:
    username, setUsername = createSignal("")
    status_msg, setStatusMsg = createSignal("")

    return Container(
        Input(bind=username, placeholder="Enter name..."),
        Button(
            "Save",
            on_click=save_user(username()).then(lambda res: setStatusMsg(res.message)),
        ),
        Show(Text(status_msg()), when=status_msg() != ""),
    )

How it works

1. @server_action registers the Python function in _ACTION_REGISTRY
2. save_user(username()) generates a ServerActionCall AST node
3. .then(lambda res: setStatusMsg(res.message)) chains a Promise handler
4. Compiled JS: fetch('/_railui_action/save_user', {method:'POST', body:JSON.stringify([sig_1()])}).then(r=>r.json()).then(res=>{ set_sig_2(res.message) })
5. The dev server (or production FastAPI server) receives the POST, deserializes the args, and calls the registered Python function

Deployment with Server Actions

Server actions require a Python backend. Deploy to:

• Railway (recommended) — railui build --platform railway generates config
• Any VPS — run uvicorn railui.backend.server:create_app --factory
• NOT Vercel — Vercel's serverless functions can't run a persistent Python process

---

Lifecycle Hooks

on_mount(log("Page mounted!"))
on_destroy(log("Page destroyed — clean up timers!"))

# Alternatively inside Show/Each:
Show(
    Text("Content"),
    when=is_visible(),
    on_mount=log("First time visible"),
    on_update=log("Visibility changed"),
    on_unmount=log("Now hidden"),
)

---

Animations

12 pre-built Web Animations API helpers:

fade_in("element-id")
fade_out("element-id")
slide_in_left("element-id", distance="40px")
slide_in_right("element-id")
slide_in_up("element-id")
slide_out_down("element-id")
spin("element-id", duration=800, iterations="Infinity")
bounce("element-id", height="12px", duration=600)
pulse("element-id", duration=800)
shake("element-id", duration=400)              # form errors
scale_in("element-id", from_scale=0.8)
scale_out("element-id", to_scale=0.8)

Chain with runSequence:

runSequence(setCount(count() + 1), shake("counter-card"))
runSequence(setCount(0), fade_out("card"), set_timeout(slide_in_up("card"), 300))

Custom keyframes:

animate("my-element", [
    {"transform": "translateX(0px)", "opacity": 1},
    {"transform": "translateX(100px)", "opacity": 0},
], duration=500, easing="ease-in-out")

---

Layout System — Named Slots

Define a layout with Slot placeholders:

# layout.py
from railui.all import *

def Layout(*children):
    return Container(
        Navbar(),
        Slot("hero", source=children, default=""),
        Container(
            Slot("body", source=children, default=Slot.Unassigned(children)),
        ),
        Slot("footer", source=children,
             default=Text("© 2026", class_name="text-gray-500")),
        class_name="min-h-screen bg-gray-50",
    )

Fill slots from pages:

# pages/index.py
def page():
    return Layout(
        SlotFill("hero", Container(
            Text("Welcome!", class_name="text-3xl font-bold text-white"),
            class_name="bg-gradient-to-r from-purple-600 to-blue-500 p-12",
        )),
        Container(
            Text("Main content here"),
            class_name="p-8",
        ),
        SlotFill("footer", Container(
            Text("Custom footer"),
        )),
    )

Slot.Unassigned(children) returns all children that are not SlotFill instances — perfect for default body content.

---

Head / Meta Tags (Per-Route)

Head(
    title="Dashboard | My App",
    meta={"description": "Dashboard page", "keywords": "python, web"},
    styles=["https://cdn.example.com/styles.css"],
    scripts=["https://cdn.example.com/widget.js"],
)

---

Suspense & Async Loading

Suspense(
    Container(
        Text(user_data().name),
        Text(user_data().email),
    ),
    fallback=Container(
        Text("Loading...", class_name="animate-pulse"),
    ),
    loading=loading(),   # boolean signal from useFetch
)

---

Form Handling

Form(
    Container(
        Label("Name", for_="name-input"),
        Input(
            bind=user_store.name,
            id="name-input",
            class_name="w-full p-3 border rounded",
        ),
    ),
    Container(
        Label("Role", for_="role-input"),
        Input(
            bind=user_store.role,
            id="role-input",
        ),
    ),
    Button("Save", type="submit",
           on_click=log("Saved:", user_store.name(), user_store.role())),
    class_name="flex flex-col gap-4 max-w-md",
)

---

JS Namespace Wrappers

Math.floor(count() / 2)
Math.random()
Math.max(count(), 0)

JSON.stringify(data())
JSON.parse(text())

Object.keys(user_data())
Object.entries(user_data())

String(count())
Number("42")

document.getElementById("my-el")
window.alert("Hello!")

Array.append(tasks(), new_task)
Array.prepend(items(), "first")
Array.remove(items(), index)
Array.concat(list_a(), list_b())

---

DOM Utilities

log("message")                   # console.log
alert("Hello!")                  # window.alert
set_timeout(log("done"), 1000)   # setTimeout
set_interval(log("tick"), 500)   # setInterval

navigate("/dashboard")           # SPA navigation
go_back()
reload()

focus_element("input-id")
scroll_to_element("section-2")

add_class("el", "highlight")
remove_class("el", "highlight")
toggle_class("el", "active")

set_inner_text("output", result())
set_inner_html("container", "<p>HTML</p>")
set_value("input", "new value")
get_value("input")

show_element("loading-spinner")
hide_element("loading-spinner")

local_storage_set("key", data())
local_storage_get("key")
local_storage_remove("key")
local_storage_clear()

copy_to_clipboard(text())

---

Conditional Classes

Container(
    Text("Error message"),
    class_list={
        "bg-red-100 text-red-800": has_error(),
        "bg-green-100 text-green-800": ~has_error(),
    },
)

Hover / Active Classes

Button(
    "Hover me",
    class_name="px-4 py-2 bg-blue-600 text-white rounded",
    hover_class="bg-blue-700 shadow-lg",
    active_class="bg-blue-800 scale-95",
)

---

CSS / Tailwind-like Utilities

RailUI compiles Tailwind-like class names to CSS at build time. 150+ utilities supported:

Category	Examples
Layout	flex, grid, block, hidden, flex-row, flex-col, items-center, justify-between
Grid	grid-cols-3, grid-rows-2, col-span-2, gap-4
Sizing	w-full, w-64, h-32, max-w-md, min-h-screen
Spacing	p-4, px-6, py-2, m-2, mx-auto, mt-8, -ml-2
Typography	text-sm, text-lg, text-4xl, font-bold, font-mono, text-center, text-red-500
Colors	bg-blue-600, text-white, border-gray-200, bg-gradient-to-r
Borders	border, border-2, border-t, rounded, rounded-lg, rounded-full
Shadows	shadow, shadow-sm, shadow-md, shadow-lg
Position	static, fixed, absolute, relative, sticky, z-50, top-0
Effects	opacity-50, transition, outline-none
Pseudo	hover:bg-blue-700, focus:outline-none, focus:ring-2
Custom	w-[100px], text-[#ff6600], bg-[#1a1a2e]

---

CLI Reference

railui dev

railui dev [path] [--host HOST] [--port PORT] [--no-open] [--platform vercel]

Starts the dev server with HMR. Builds into .railui/dev/ — your project stays clean.

• File watcher rebuilds on .py changes
• SSE pushes reload event to browser
• Serves from .railui/dev/ with SPA fallback

railui build

railui build [path] [--outdir DIR] [--no-bundle] [--platform railway|vercel]

Production build. Generates:

• index.html — SPA shell
• app.{hash}.js — Minified JS bundle
• app.{hash}.css — Compiled Tailwind CSS
• Deployment configs (if --platform is set)

railui new

railui new <project-name> [--dir PARENT_DIR]

Scaffolds a new project with main.py, layout.py, pages/index.py, and config.

---

Configuration

{
  "outdir": "build",
  "port": 5173,
  "open_browser": true,
  "bundle": true,
  "platform": "railway",
  "public_dirs": ["public", "assets"]
}

railui.config.json in your project root. CLI flags override config values.

---

Deployment

Railway (Full-Stack — Server Actions Supported)

railui build --platform railway
railway up

Railway runs the FastAPI backend via uvicorn. Server actions work out of the box.

The deployment templates (railway.toml, Procfile, requirements.txt) are auto-generated.

Vercel (Static SPA — No Server Actions)

railui build --platform vercel
vercel --prod

Vercel cannot run Python server actions. The SPA itself works perfectly (routing, signals, animations, etc.), but @server_action calls return 404.

Use Vercel for:

• Marketing sites
• Dashboards with client-only data (via useFetch on public APIs)
• Prototypes

Any Static Host

Just upload the dist/ directory after railui build.

Docker / Custom Server

FROM python:3.11-slim
WORKDIR /app
COPY . .
RUN pip install railui && railui build
CMD ["uvicorn", "railui.backend.server:create_app", "--factory", "--host", "0.0.0.0", "--port", "8000"]

---

Architecture Overview

Python AST (DSL)          Compile Time              Browser (Runtime)
─────────────────      ──────────────────      ──────────────────────
count + 1              →  sig_1() + 1          →  $signals.sig_1 + 1
setCount(42)           →  set_sig_1(42)        →  $signals.sig_1 = 42
                                                      ↓
Component.render()     →  HTML + Effects       →  runEffects()
                                                      ↓
createSignal(0)        →  createSignal(...)    →  DOM updates
                                                      ↓
@server_action           fetch(...).then(...)  →  FastAPI RPC

Key Design Decisions

Decision	Why
Compile-time DSL	Zero runtime overhead — no interpreter shipped to browser
Direct DOM manipulation	No virtual DOM — signals update elements directly via innerText, classList, etc.
Push-based signals	setSignal(val) triggers runEffects() — simple, deterministic
Pre-compiled routes	All route HTML is inlined in JS — no dynamic server rendering
Tailwind-like CSS	Regex-based compiler — 150+ rules, zero runtime CSS-in-JS
FastAPI backend	Async Python server for dev, HMR, and server actions
~1KB runtime JS	Minimal client — faster loads, less to debug

---

Full API Reference

Core Types

Symbol	Description
DSLExpr	Base class for all DSL expressions
RawJS(js)	Inject raw JavaScript string
to_dsl(val)	Convert Python value to DSLExpr

Signals & State

Symbol	Description
createSignal(initial) → (getter, setter)	Create a reactive signal
createStore(dict) → Store	Create grouped signals
useComputed(fn) → DSLExpr	Derived reactive expression
createEffect(expr)	Register side effect
useFetch(url, on_success, ...)	Async data fetching

App

Symbol	Description
App(title)	Application instance
app.route(path)	Decorator — register route
app.discover_pages(dir)	File-based routing
app.set_not_found(factory)	404 page
app.build(out_dir)	Compile bundle

Components

Component	Tag	Key Props
Container	<div>	class_name, on_click, ...
Text	<span>	class_name
Button	<button>	type, disabled, on_click
Input	<input />	type, value, placeholder, bind, on_input
Textarea	<textarea>	rows, cols, bind
Select	<select>	name, bind, on_change
Option	<option>	value, selected
Label	<label>	for_
Form	<form>	action, method, on_submit
Link	<a>	href (required), target
Image	<img />	src, alt (required)
Page	<main>	semantic main element
Show	<div>	when (required), fallback, lifecycle
Each	<div>	items (required), render_fn (required), lifecycle
Head	—	title, meta, styles, scripts
Suspense	<div>	fallback, loading
Slot	—	name, source, default
SlotFill	—	name

Server Actions

Symbol	Description
@server_action	Decorator — register server-side function
ServerActionCall.then(callback)	Chain Promise handler

Animations

Symbol	Description
animate(id, keyframes, **opts)	Custom WAAPI animation
fade_in(id) / fade_out(id)	Opacity fade
slide_in_left/right/up(id)	Slide + fade in
slide_out_down(id)	Slide out
spin(id)	Continuous rotation
bounce(id)	Vertical bounce
pulse(id)	Opacity pulse
shake(id)	Horizontal shake
scale_in(id) / scale_out(id)	Scale transition

JS Namespaces

Math, JSON, Object, String, Number, Boolean, window, document, Array

Utility Functions

log, warn, error, alert, confirm_dialog, set_timeout, set_interval, clear_interval, clear_timeout, navigate, go_back, go_forward, reload, open_url, focus_element, blur_element, click_element, scroll_to, scroll_to_element, scroll_to_top, set_attribute, remove_attribute, add_class, remove_class, toggle_class, set_inner_text, set_inner_html, set_value, get_value, set_style, show_element, hide_element, local_storage_set, local_storage_get, local_storage_remove, local_storage_clear, session_storage_set, session_storage_get, session_storage_remove, copy_to_clipboard, on_mount, on_destroy, runSequence, ifelse, typeof, event_value, prevent_default, stop_propagation

---

Project Structure

my-app/
├── main.py              # App entry — App(), discover_pages()
├── layout.py            # Shared layout with Slot system
├── store.py             # Global state (createStore)
├── railui.config.json   # Framework config
├── pages/
│   ├── index.py         # Route: /
│   ├── dashboard.py     # Route: /dashboard
│   └── users/
│       ├── index.py     # Route: /users
│       └── [id].py      # Route: /users/:id
├── public/              # Static assets
│   └── favicon.ico
└── dist/                # Build output (after `railui build`)
    ├── index.html
    ├── app.{hash}.js
    └── app.{hash}.css

---

License

MIT