---
name: reflex-dev
description: Guide for building full-stack web applications using Reflex, a Python framework that compiles to React frontend and FastAPI backend. Use when creating, modifying, or debugging Reflex apps - covers state management, event handlers, components, routing, styling, and data integration patterns.
---

# Reflex Development

## Overview

Reflex is a full-stack Python framework for building web applications without writing JavaScript. Apps compile to a React frontend and FastAPI backend, with state management and event handlers running entirely in Python.

**Architecture:**
- **Frontend**: Compiled to React (JavaScript) for UI rendering
- **Backend**: FastAPI server running Python event handlers
- **Communication**: WebSockets for real-time state updates
- **State**: Server-side Python state synchronized to frontend

## Core Concepts

### State Management

State is a Python class that holds all mutable data and event handlers. All state variables must be JSON-serializable.

```python
import reflex as rx

class AppState(rx.State):
    # State variables (any JSON-serializable type)
    count: int = 0
    items: list[str] = []
    user_name: str = ""

    # Event handlers - the ONLY way to modify state
    def increment(self):
        self.count += 1

    def add_item(self, item: str):
        self.items.append(item)

    # Computed vars (derived state)
    @rx.var
    def item_count(self) -> int:
        return len(self.items)
```

**Key Rules:**
- State vars MUST be JSON-serializable (int, str, list, dict, bool, float)
- Only event handlers can modify state
- Use `@rx.var` decorator for computed/derived values
- State is per-user session (isolated between users)

### Components

Components are UI building blocks. Reflex provides 60+ built-in components.

```python
import reflex as rx

def header() -> rx.Component:
    return rx.heading("My App", size="lg")

def counter_component(state: AppState) -> rx.Component:
    return rx.vstack(
        rx.text(f"Count: {state.count}"),
        rx.button("Increment", on_click=state.increment),
        spacing="4"
    )
```

**Common Components:**
- Layout: `rx.vstack`, `rx.hstack`, `rx.box`, `rx.container`
- Text: `rx.heading`, `rx.text`, `rx.code`
- Input: `rx.input`, `rx.text_area`, `rx.select`, `rx.checkbox`
- Interactive: `rx.button`, `rx.link`, `rx.icon_button`
- Data: `rx.table`, `rx.data_table`, `rx.list`
- Charts: `rx.recharts.line_chart`, `rx.recharts.bar_chart`, etc.

### Event Handlers

Event handlers respond to user interactions and are the ONLY way to modify state.

```python
class FormState(rx.State):
    form_data: dict[str, str] = {}

    # Simple event handler
    def handle_submit(self):
        print(f"Submitted: {self.form_data}")

    # Event handler with argument
    def update_field(self, field: str, value: str):
        self.form_data[field] = value

    # Async event handler (for API calls, DB queries)
    async def fetch_data(self):
        # Can use any Python library
        import httpx
        async with httpx.AsyncClient() as client:
            response = await client.get("https://api.example.com/data")
            self.data = response.json()
```

**Event Triggers** (connect components to handlers):
- `on_click`: Button clicks
- `on_change`: Input field changes
- `on_submit`: Form submissions
- `on_mount`: Component first renders
- `on_blur`, `on_focus`: Input focus events

## Project Structure

Standard Reflex app structure:

```
my_app/
├── my_app/
│   ├── __init__.py         # Empty
│   └── my_app.py           # Main app file (State + pages)
├── assets/                 # Static files (images, fonts, etc.)
├── .web/                   # Auto-generated frontend (don't edit)
├── rxconfig.py             # Reflex configuration
└── requirements.txt        # Python dependencies
```

### Main App File Pattern

```python
import reflex as rx

# 1. Define State
class State(rx.State):
    count: int = 0

    def increment(self):
        self.count += 1

# 2. Define Pages
def index() -> rx.Component:
    return rx.container(
        rx.heading("Welcome"),
        rx.button("Click", on_click=State.increment),
        rx.text(f"Count: {State.count}")
    )

def about() -> rx.Component:
    return rx.container(
        rx.heading("About"),
        rx.link("Home", href="/")
    )

# 3. Create App and Add Routes
app = rx.App()
app.add_page(index, route="/")
app.add_page(about, route="/about")
```

## Common Patterns

### Form Handling

```python
class FormState(rx.State):
    name: str = ""
    email: str = ""

    def handle_submit(self, form_data: dict):
        self.name = form_data.get("name", "")
        self.email = form_data.get("email", "")

def form_page():
    return rx.form(
        rx.vstack(
            rx.input(name="name", placeholder="Name"),
            rx.input(name="email", placeholder="Email"),
            rx.button("Submit", type="submit"),
        ),
        on_submit=FormState.handle_submit,
    )
```

### Data Tables

```python
class DataState(rx.State):
    data: list[dict] = [
        {"id": 1, "name": "Alice", "age": 25},
        {"id": 2, "name": "Bob", "age": 30},
    ]

def data_table_page():
    return rx.data_table(
        data=DataState.data,
        columns=["id", "name", "age"],
        sort=True,
        search=True,
        pagination=True,
    )
```

### File Upload

```python
class UploadState(rx.State):
    async def handle_upload(self, files: list[rx.UploadFile]):
        for file in files:
            upload_data = await file.read()
            # Process file data
            outfile = f"./uploads/{file.filename}"
            with open(outfile, "wb") as f:
                f.write(upload_data)

def upload_page():
    return rx.vstack(
        rx.upload(
            rx.button("Select Files"),
            id="upload1",
        ),
        rx.button(
            "Upload",
            on_click=UploadState.handle_upload(rx.upload_files(upload_id="upload1"))
        ),
    )
```

### Database Integration (with DuckDB)

```python
import duckdb
import polars as pl

class DBState(rx.State):
    records: list[dict] = []

    async def load_data(self):
        # Use existing database connection
        conn = duckdb.connect("data/mydb.duckdb")
        df = conn.execute("SELECT * FROM mytable").pl()
        self.records = df.to_dicts()
        conn.close()

    async def insert_record(self, data: dict):
        conn = duckdb.connect("data/mydb.duckdb")
        conn.execute(
            "INSERT INTO mytable (name, value) VALUES (?, ?)",
            [data["name"], data["value"]]
        )
        conn.close()
        await self.load_data()  # Refresh
```

## Styling & Layout

### Inline Styling

```python
rx.box(
    rx.text("Styled text"),
    bg="#1a5f9e",
    color="white",
    padding="4",
    border_radius="md",
)
```

### Responsive Layout

```python
rx.container(
    rx.responsive_grid(
        rx.box("Item 1", bg="blue"),
        rx.box("Item 2", bg="green"),
        rx.box("Item 3", bg="red"),
        columns=[1, 2, 3],  # 1 col mobile, 2 tablet, 3 desktop
        spacing="4",
    ),
    max_width="1200px",
)
```

### Common Style Props

- Layout: `width`, `height`, `padding`, `margin`, `display`
- Colors: `bg` (background), `color` (text)
- Typography: `font_size`, `font_weight`, `text_align`
- Borders: `border`, `border_radius`, `border_color`
- Spacing: `spacing` (for stacks), `gap`

## Routing

### Multiple Pages

```python
app = rx.App()

# Route with parameters
@rx.page(route="/user/[id]")
def user_page() -> rx.Component:
    return rx.text(f"User ID: {State.router.page.params.get('id')}")

# Simple routes
app.add_page(index, route="/")
app.add_page(about, route="/about")
```

### Navigation

```python
# Links
rx.link("Go to About", href="/about")

# Programmatic navigation
def go_home(self):
    return rx.redirect("/")
```

## Development Workflow

### Initialize New App

```bash
pip install reflex
reflex init
```

### Run Development Server

```bash
reflex run
```

App runs on `http://localhost:3000` with auto-reload.

### Common Commands

```bash
reflex run          # Start dev server
reflex export       # Build production bundle
reflex db init      # Initialize database (if using Reflex DB)
reflex db migrate   # Run migrations
```

## Best Practices

1. **State Organization**: Split large states into substates
   ```python
   class AuthState(rx.State):
       user: str = ""

   class DataState(rx.State):
       items: list = []
   ```

2. **Component Reusability**: Create reusable component functions
   ```python
   def card(title: str, content: str) -> rx.Component:
       return rx.box(
           rx.heading(title, size="md"),
           rx.text(content),
           padding="4",
           border="1px solid #ddd",
       )
   ```

3. **Event Handler Performance**: Use async for I/O operations
   ```python
   async def fetch_data(self):
       # Async I/O won't block other users
       self.data = await some_api_call()
   ```

4. **Type Hints**: Always type-hint state vars and event handlers
   ```python
   count: int = 0
   items: list[str] = []

   def update_count(self, value: int) -> None:
       self.count = value
   ```

## References

### Documentation
- Official Docs: https://reflex.dev/docs/getting-started/introduction/
- Component Library: https://reflex.dev/docs/library
- Tutorials: https://reflex.dev/docs/getting-started/tutorial/

### Example Apps
See `examples/` directory for complete working examples:
- Simple counter app
- Data table with CRUD operations
- Form with validation
- File upload and processing

### Common Patterns Reference
See `references/patterns.md` for detailed examples of:
- Authentication flows
- Real-time updates
- Complex form validation
- Multi-step workflows
- Data visualization with charts