python-flet

Python Flet Development

Flet is a Python framework for building cross-platform web, desktop, and mobile applications without prior frontend experience. It wraps Flutter widgets and exposes them as Python controls.

App Entry Point

Every Flet app has a main function receiving a ft.Page and ends with ft.run(main):

import flet as ft

def main(page: ft.Page):
    page.title = "My App"
    page.add(ft.Text("Hello, Flet!"))

ft.run(main)

Installation: pip install 'flet[all]'

Run desktop: flet run main.py

Run web: flet run --web main.py

Hot reload (watch directory recursively): flet run --recursive main.py

Project Structure

my-app/
├── pyproject.toml
└── src/
    ├── assets/
    │   └── icon.png
    └── main.py

Create via: flet create (or uv run flet create)

Core Concepts

Page

ft.Page is the root container. Configure it before adding controls:

def main(page: ft.Page):
    page.title = "App Title"
    page.theme_mode = ft.ThemeMode.LIGHT
    page.theme = ft.Theme(color_scheme_seed=ft.Colors.BLUE)
    page.vertical_alignment = ft.MainAxisAlignment.CENTER
    page.horizontal_alignment = ft.CrossAxisAlignment.CENTER
    page.scroll = ft.ScrollMode.AUTO

Controls and update()

Controls are Python objects. Mutations require page.update() or control.update():

text = ft.Text("Initial")
page.add(text)

def change(e):
    text.value = "Changed"
    text.update()          # prefer: update only this control
    # page.update()        # fallback: updates entire page

Rule: Prefer control.update() over page.update() — sends a smaller diff.

Layout Controls

Control	Purpose
ft.Column	Vertical stack
ft.Row	Horizontal stack
ft.Stack	Absolute positioning
ft.Container	Box with padding, color, border, animation
ft.ListView	Efficient vertical/horizontal scrollable list
ft.GridView	Efficient scrollable grid

page.add(
    ft.Row(
        controls=[ft.Text("Left"), ft.Text("Right")],
        alignment=ft.MainAxisAlignment.SPACE_BETWEEN,
    )
)

Expand: Use expand=True to fill available space:

ft.ListView(expand=True, spacing=10)

Custom Controls

Styled Controls (inherit a single Flet control)

@ft.control
class PrimaryButton(ft.Button):
    bgcolor: ft.Colors = ft.Colors.BLUE_700
    color: ft.Colors = ft.Colors.WHITE

Use @ft.control or @dataclass decorator. Field types must be annotated.

Composite Controls (combine multiple controls)

@ft.control
class TaskItem(ft.Row):
    text: str = ""

    def init(self):
        self.checkbox = ft.Checkbox()
        self.label = ft.Text(value=self.text)
        self.controls = [self.checkbox, self.label]

    def toggle(self, e):
        self.label.color = ft.Colors.GREY if self.checkbox.value else None
        self.update()

Lifecycle Methods

Method	When called	Use for
init()	After __init__	Setup sub-controls
build()	When assigned self.page	Platform-dependent logic
did_mount()	After added to page	Start timers, fetch data
will_unmount()	Before removed	Clean up, cancel tasks
before_update()	Every update()	Sync derived state

Isolation rule: Any custom control that calls self.update() inside its own methods must set is_isolated = True (or override is_isolated property to return True).

Navigation and Routing

Use page.on_route_change + page.views as the single source of truth:

def main(page: ft.Page):
    def route_change():
        page.views.clear()
        page.views.append(ft.View("/", controls=[...]))
        if page.route == "/settings":
            page.views.append(ft.View("/settings", controls=[...]))
        page.update()

    async def view_pop(e):
        page.views.remove(e.view)
        await page.push_route(page.views[-1].route)

    page.on_route_change = route_change
    page.on_view_pop = view_pop
    route_change()

ft.run(main)

Navigate: await page.push_route("/settings")

Parameterized routes:

troute = ft.TemplateRoute(page.route)
if troute.match("/items/:id"):
    print(troute.id)

Routing rules:

• Always keep a root / view in page.views
• Centralize all route logic in page.on_route_change
• Always handle page.on_view_pop to stay in sync

Theming

# App-wide theme
page.theme = ft.Theme(color_scheme_seed=ft.Colors.GREEN)
page.dark_theme = ft.Theme(color_scheme_seed=ft.Colors.BLUE)
page.theme_mode = ft.ThemeMode.SYSTEM  # LIGHT | DARK | SYSTEM

# Nested theme (scoped to a container)
ft.Container(
    theme=ft.Theme(color_scheme=ft.ColorScheme(primary=ft.Colors.PINK)),
    content=ft.Button("Pink button"),
)

Async Apps

Mark main async for asyncio support. Use asyncio.sleep() (not time.sleep()).

import asyncio
import flet as ft

async def main(page: ft.Page):
    async def on_click(e):
        await asyncio.sleep(1)
        page.add(ft.Text("Done!"))

    page.add(ft.Button("Click me", on_click=on_click))

ft.run(main)

Use page.run_task(coro) to run background coroutines from did_mount().

Performance: Large Lists

Use ListView or GridView instead of Column/Row for hundreds of items:

# Efficient: renders only visible items
lv = ft.ListView(expand=True, spacing=10, item_extent=50)
for i in range(5000):
    lv.controls.append(ft.Text(f"Line {i}"))
page.add(lv)

# Batch updates to avoid large WebSocket messages
for i in range(5000):
    lv.controls.append(ft.Text(f"Line {i}"))
    if i % 500 == 0:
        page.update()
page.update()

Implicit Animations

Enable by setting animate_* properties on controls:

# Opacity fade
container = ft.Container(
    width=150, height=150,
    bgcolor=ft.Colors.BLUE,
    animate_opacity=300,  # ms
)

# Scale with bounce curve
ft.Container(
    animate_scale=ft.Animation(
        duration=600,
        curve=ft.AnimationCurve.BOUNCE_OUT,
    )
)

# Position animation (inside Stack or page.overlay)
ft.Container(animate_position=1000)

# Animated content switcher
ft.AnimatedSwitcher(
    content,
    transition=ft.AnimatedSwitcherTransition.SCALE,
    duration=500,
)

Window Control (Desktop)

page.title = "My App"
page.window.width = 800
page.window.height = 600
page.window.resizable = True
page.window.always_on_top = False
page.window.center()

Quick Reference: Common Controls

ft.Text("Hello", size=20, weight=ft.FontWeight.BOLD)
ft.Button("Click", on_click=handler)
ft.IconButton(ft.Icons.ADD, on_click=handler)
ft.TextField(label="Name", on_change=handler)
ft.Checkbox(label="Check me", on_change=handler)
ft.Dropdown(options=[ft.dropdown.Option("A"), ft.dropdown.Option("B")])
ft.Image(src="photo.jpg")                   # from assets/
ft.Image(src="https://example.com/img.png") # remote
ft.AppBar(title=ft.Text("Title"))
ft.NavigationDrawer(controls=[...])
ft.AlertDialog(title=ft.Text("Alert"), content=ft.Text("Body"))

Additional Resources

Reference Files

• references/controls-and-layout.md - Complete layout patterns, Container styling, Stack positioning, and control properties
• references/patterns-and-best-practices.md - State management, session/client storage, PubSub, keyboard shortcuts, error handling

Example Files

• examples/counter-app.py - Minimal working counter demonstrating page setup and event handlers
• examples/custom-control.py - Composite custom control with lifecycle methods and isolation