PyWebWinUI3

PyWebWinUI3 is a Windows desktop UI framework that combines:

• a Python backend
• a pywebview host window
• a Svelte frontend rendered inside Edge WebView2
• an XAML-like page format for declaring UI

The goal is to make desktop apps feel like WinUI3-style apps while still being driven from Python.

What It Does

PyWebWinUI3 lets you:

• declare pages in XAML-like markup
• bind controls to Python values
• react to value changes and lifecycle events from Python
• serve a bundled frontend from the Python runtime
• keep a Windows-style shell with theme and accent synchronization

At runtime, Python owns the application state, pywebview hosts the window, and the Svelte app renders the page tree loaded from the bundled frontend.

Platform

PyWebWinUI3 is currently aimed at Windows.

Runtime dependencies:

• pywebview
• hPyT

Installation

pip install PyWebWinUI3

Quick Start

from pywebwinui3.core import MainWindow
from pywebwinui3.type import Status

app = MainWindow("PyWebWinUI3")

app.addSettings("Settings.xaml")
app.addPage("Dashboard.xaml")
app.addPage("Test.xaml")

app.values["test_input"] = "Hello, world"
app.values["test_switch"] = False

@app.onValueChange("test_noticeSample")
def show_notice(*_):
    app.notice(Status.Attention, "Title", "This is a sample notice")

app.start()

Minimal Page Example

<Page path="settings" icon="&#xE713;" name="Settings" title="Settings">
	<Box>
		<Horizontal>
			<Text>App theme</Text>
			<Space />
			<Select value="system_theme">
				<Option value="dark">Dark</Option>
				<Option value="light">Light</Option>
				<Option value="system">Use system setting</Option>
			</Select>
		</Horizontal>
	</Box>
</Page>

Core Concepts

MainWindow

pywebwinui3.core.MainWindow is the main entry point.

Important methods:

• addPage(...)
• addSettings(...)
• start(debug=False, hidden=False, on_top=None, width=None, height=None, min_width=900, min_height=600)
• notice(level, title, description, item=None)
• pin(state)
• syncValue(key, value)

values

app.values is the shared state dictionary between Python and the frontend.

Typical usage:

app.values["user_name"] = "Haruna"
app.values["is_enabled"] = True
app.values["progress"] = 50

The frontend reads these values directly, and user input writes back into the same store.

Built-in system values include keys such as:

• system_title
• system_icon
• system_theme
• system_accent
• system_pages
• system_settings
• system_nofication
• system_pin

Events

MainWindow exposes decorator-style event hooks:

• @app.onValueChange(key)
• @app.onAccentColorChange()
• @app.onSetup()
• @app.onExit()

Example:

@app.onValueChange("user_name")
def on_user_name_change(*_):
    print(app.values["user_name"])

Notifications

Use app.notice(...) to push InfoBar-style messages into the frontend.

from pywebwinui3.type import Status

app.notice(Status.Success, "Saved", "Settings were saved successfully")

Status values:

• Status.Attention
• Status.Success
• Status.Caution
• Status.Critical
• Status.Neutral

Markup Model

Pages are loaded into a JSON tree and rendered by the frontend.

Common tags currently implemented:

• Page
• Box
• Horizontal
• Vertical
• Text
• Button
• Input
• Slider
• Switch
• Check
• Radio
• Select
• Option
• Image
• Webview
• Progressbar
• Line
• Space
• Expender
• If
• Match
• Repeat

Binding Model

PyWebWinUI3 supports two main binding styles:

• direct value binding through attributes like value="test_input"
• formatted text / expressions inside strings such as {user_name}

The frontend compiles and caches formatted expressions, while Python remains the source of truth for state.

Path-like targets such as nested array access are also handled by the runtime value system when used by supported controls.

Resource Resolution

Relative resource paths are resolved from the Python caller's directory first, then from the packaged frontend bundle directory.

This lets you write things like:

• local XAML files via addPage("Dashboard.xaml")
• local images in markup
• bundled web assets shipped with the package

External URLs such as https://... and file://... are passed through as-is.

Window / Shell Behavior

The desktop shell is implemented with pywebview and provides:

• a hidden-title-bar host window
• theme and accent synchronization with Windows
• always-on-top pinning
• a pywebview API bridge between Python and the frontend
• direct bundled frontend hosting from pywebwinui3/web/

Project Structure

PyWebWinUI3/
├─ pywebwinui3/
│  ├─ core.py        # public API, app state, pywebview host
│  ├─ util.py        # XAML loader, sync dict, accent watcher
│  ├─ event.py       # event system
│  ├─ type.py        # status and theme resource constants
│  └─ web/           # built frontend bundle shipped to users
├─ frontend/         # Svelte source
├─ example/          # sample application and XAML pages
└─ setup.py

Development Notes

If you are only using the package, you do not need to build the frontend manually.

If you are working on the framework itself:

• Python runtime code lives in pywebwinui3/
• Svelte source lives in frontend/
• the distributable frontend bundle lives in pywebwinui3/web/

Example

See:

• example/example.py
• example/Settings.xaml
• example/Dashboard.xaml
• example/Test.xaml

License

Apache-2.0