We've just released Flet 0.4.0 with a super exciting new feature - packaging Flet apps into a standalone static website that can be run entirely in the browser! The app can be published to any free hosting for static websites such as GitHub Pages or Cloudflare Pages. Thanks to Pyodide - a Python port to WebAssembly!

You can quickly build awesome single-page applications (SPA) entirely in Python and host them everywhere! No HTML, CSS or JavaScript required!

Quick Flet with Pyodide demo​

Install the latest Flet package:

pip install flet --upgrade

Create a simple counter.py app:

import flet as ft

def main(page: ft.Page):
    page.title = "Flet counter example"
    page.vertical_alignment = ft.MainAxisAlignment.CENTER

    txt_number = ft.TextField(value="0", text_align=ft.TextAlign.RIGHT, width=100)

    def minus_click(e):
        txt_number.value = str(int(txt_number.value) - 1)
        page.update()

    def plus_click(e):
        txt_number.value = str(int(txt_number.value) + 1)
        page.update()

    page.add(
        ft.Row(
            [
                ft.IconButton(ft.icons.REMOVE, on_click=minus_click),
                txt_number,
                ft.IconButton(ft.icons.ADD, on_click=plus_click),
            ],
            alignment=ft.MainAxisAlignment.CENTER,
        )
    )

ft.app(main)

Run a brand-new flet publish command to publish Flet app as a static website:

flet publish counter.py

The website will be published to dist directory next to counter.py. Give website a try using built-in Python web server:

python -m http.server --directory dist

Open http://localhost:8000 in your browser to check the published app.

Here are a few live Flet apps hosted at Cloudflare Pages:

To-Do

Icons browser

Calc

Solitaire

Trolli

Check the guide for more information about publishing Flet apps as standalone websites.

Built-in Fletd server in Python​

Flet 0.4.0 also implements a new Flet desktop architecture.

It replaces Fletd server written in Go with a light-weight shim written in Python with a number of pros:

1. Only 2 system processes are needed to run Flet app: Python interpreter and Flutter client.
2. Less communication overhead (minus two network hops between Python and Fletd) and lower latency (shim uses TCP on Windows and Unix domain sockets on macOS/Linux).
3. Shim binds to 127.0.0.1 on Windows by default which is more secure.
4. The size of a standalone app bundle produced by flet pack reduced by ~8 MB.

The implementation was also required to support Pyodide (we can't run Go web server in the browser, right?) and paves the way to iOS and Android support.

Other changes​

• All controls loading resources from web URLs (Image.src, Audio.src, Page.fonts, Container.image_src) are now able to load them from local files too, by providing a full path in the file system, and from assets directory by providing relative path. For desktop apps a path in src property could be one of the following:
  • A path relative to assets directory, with or without starting slash, for example: /image.png or image.png. The name of artifact dir should not be included.
  • An absolute path within a computer file system, e.g. C:\projects\app\assets\image.png or /Users/john/images/picture.png.
  • A full URL, e.g. https://mysite.com/images/pic.png.
  • Add page.on_error = lambda e: print("Page error:", e.data) to see failing images.
• flet Python package has separated into two packages: flet-core and flet.
• PDM replaced with Poetry.
• beartype removed everywhere.

💥 Breaking changes​

• Default routing scheme changed from "hash" to "path" (no /#/ at the end of app URL). Use ft.app(main, route_url_strategy="hash") to get original behavior.
• OAuth authentication is not supported anymore in standalone desktop Flet apps.

Async support​

Flet apps can now be written as async apps and use asyncio with other Python async libraries. Calling coroutines is naturally supported in Flet, so you don't need to wrap them to run synchronously.

To start with an async Flet app you should make main() method async:

import flet as ft

async def main(page: ft.Page):
    await page.add_async(ft.Text("Hello, async world!"))

ft.app(main)

Read the guide for more information about writing async Flet apps.

Conclusion​

Flet 0.4.0 brings the following exciting features:

• Standalone web apps with Pyodide running in the browser and hosted on a cheap hosting.
• Faster and more secure architecture with a built-in Fletd server.
• Async apps support.

Upgrade Flet module to the latest version (pip install flet --upgrade), give flet publish command a try and let us know what you think!

Hey, by the way, Flet project has reached ⭐️ 4.2K stars ⭐️ (+1K in just one month) - keep going!

Happy New Year! Flet project has reached ⭐️ 3.3K stars ⭐️ on GitHub which is very exciting and encouraging! Thank you all for your support!

We are starting this year with the release of Flet 0.3.2 bringing a long-awaited feature: creating standalone desktop bundles with a custom icon!

flet command has been used for running Flet program with hot reload, but we recently re-worked Flet CLI to support multiple actions.

There is a new flet pack command that wraps PyInstaller API to package your Flet Python app into a standalone Windows executable or macOS app bundle which can be run by a user with no Python installed.

Command's --icon argument is now changing not only executable's icon, but Flet's app window icon and the icon shown in macOS dock, Windows taskbar, macOS "About" dialog, Task Manager and Activity Monitor:

Bundle name, version and copyright can be changed too:

Find all available options for packaging desktop apps in the updated guide.

Upgrade Flet module to the latest version (pip install flet --upgrade), give flet pack command a try and let us know what you think!

This post is a continuation of Flet mobile strategy published a few months ago.

Our original approach to Flet running on a mobile device was Server-Driven UI. Though SDUI has its own benefits (like bypassing App Store for app updates) it doesn't work in all cases, requires web server to host Python part of the app and, as a result, adds latency which is not great for user actions requiring nearly instance UI response, like drawing apps.

I've been thinking on how to make Python runtime embedded into Flutter iOS or Android app to run user Python program. No doubt, it's technically possible as Kivy and BeeWare projects do that already.

Current Flet architecture​

The current architecture of Flet desktop app is shown on the diagram below:

Running Flet program on a desktop involves three applications (processes) working together:

• Python runtime (python3) - Python interpreter running your Python script. This is what you are starting from a command line. Python starts Fletd server and connects to it via WebSockets.
• Fletd server (fletd)- Flet web server written in Go, listening on a TCP port. Fletd holds the state of all user sessions (for desktop app there is only one session) and dispatches page updates and user generated events between Python program and Flet client.
• Flet client (flet) - desktop app written in Flutter and displaying UI in a native OS window. Flet client connects to Fletd server via WebSockets.

The architecture above works well for Flet web apps where web server is essential part, but for desktop it seems redundant:

• If all three processes run on the same computer WebSockets could be replaced with sockets or named pipes with less overhead.
• Fletd server has no much sense as there is only one user session and UI state is persistently stored in Flet desktop client which is never "reloaded".

Flet new desktop architecture​

Flet desktop app architecture can be simplified by replacing Fletd with a "stub" written in Python and communicating with Flet desktop client via sockets (Windows) and named pipes (macOS and Linux):

Flet mobile architecture​

Mobile applications are running in a very strict context with a number of limitations. For example, on iOS the app cannot spawn a new processes. Other words, Flet Flutter app cannot just start "python.exe" and pass your script as an argument.

Luckily for us, Python can be embedded into another app as a C library and Dart (the language in which Flutter apps are written) allows calling C libraries via FFI (Foreign Function Interface).

Additionally, while Android allows loading of dynamically linked libraries iOS requires all libraries statically linked into app executable. This article covers Dart FFI in more details, if you are curious.

Flet mobile architecture could look like this:

Python runtime will be statically or dynamically linked with Flutter client app and called via FFI and/or named pipes.

Running Python on mobile will have some limitations though. Most notable one is the requirement to use "pure" Python modules or modules with native code compiled specifically for mobile ARM64 architecture.

Asyncio support​

Asyncio is part of Python 3 and we start seeing more and more libraries catching up with async/await programming model which is more effective for I/O-bound and UI logic.

Currently, Flet is spawning all UI event handlers in new threads and it's also a pain to see threading.sleep() calls hogging threads here and there just to do some UI animation. All that looks expensive.

Using of async libraries from a sync code is possible, but looks hacky and inefficient as it keeps CPU busy just to wait async method to finish. So, we want a first-class support of async code in Flet app.

Async/await model is a state machine switching between tasks in a single thread. By going async Flet will able to utilize streams for socket server and use async WebSockets library library. It will be possible to use both sync and async event handlers in a single Flet app without any compromises or hacks.

Even more exciting, async Flet will be able to run entirely in the browser within Pyodide - Python distribution based on WebAssembly (WASM). WebAssembly doesn't have multi-threading support yet, so running in a single thread is a must. Just imagine, Flet web app with a trully offline Flet PWA that does not require a web server to run a Python code!

Development plan​

We are going to crunch the scope above in a few iterations:

1. Async API support with async WebSockets library. Works with the same Fletd in Go.
2. Fletd server ("stub") in Python to use with a desktop.
3. Embedding Python with Fletd "stub" and user program into iOS.
4. Embedding Python into Android.
5. Packaging mobile apps for iOS and Android.

Hop to Discord to discuss the plan, offer help, ask questions!

Flet is a fast-evolving framework with a new functionality and bug fixes being committed every other day.

The development model with one pull request per release didn't work well for the project as users waited for weeks to get hands on a new release and, honestly, from development perspective producing large "heroic" releases takes a lot of energy 🫠.

From now on we'll be breaking releases into multiple pull requests with one feature/bugfix per PR.

Every PR merged into main branch will be publishing pre-release (developmental release) package to pypi.org having version format of X.Y.Z.devN.

Installing pre-releases​

To install Flet pre-release package use the following command:

pip install flet --pre

Flet versioning​

Flet is switching to Semanting Versioning with a version number MAJOR.MINOR.PATCH:

1. MAJOR will be incremented when there are "incompatible API changes". Right now it's 0 and we expect to make it 1 when we feel that Flet API is stable enough.
2. MINOR will be incremented when a new functionality added in a backwards compatible manner.
3. PATCH will be incremented when we make backward compatible bug fixes.

According to that rule, upcoming Flet release will have version 0.2.0. Bug fixes for that release will be labeled as 0.2.1, 0.2.2, etc. The release after that release will be 0.3.0 and so on.

Flet pre-releases will have a format of MAJOR.{LAST_MINOR + 1}.0.dev{BUILD} where LAST_MINOR is MINOR version of the last release and {BUILD} is a build number set by CI. For example, if the last published release is 0.1.65 pre-releases will have versions 0.2.0.dev{BUILD}. Pre-releases after 0.2.0 release will be labeled as 0.3.0.dev{BUILD}.

We just released Flet 0.1.65 which is adding a bunch of mobile-optimized controls, fixing some bugs and introducing a new layout control - ResponsiveRow.

ResponsiveRow control​

ResponsiveRow borrows the idea of grid layout from Bootstrap web framework.

ResponsiveRow allows aligning child controls to virtual columns. By default, a virtual grid has 12 columns, but that can be customized with ResponsiveRow.columns property.

Similar to expand property every control now has col property which allows specifying how many columns a control should span. For examle, to make a layout consisting of two columns spanning 6 virtual columns each:

import flet as ft

ft.ResponsiveRow([
    ft.Column(col=6, controls=ft.Text("Column 1")),
    ft.Column(col=6, controls=ft.Text("Column 2"))
])

ResponsiveRow is "responsive" because it can adapt the size of its children to a changing screen (page, window) size. col property in the example above is a constant number which means the child will span 6 columns for any screen size.

If ResponsiveRow's child doesn't have col property specified it spans the maximum number of columns.

col can be configured to have a different value for specific "breakpoints". Breakpoints are named dimension ranges:

For example, the following example collapses content into a single column on a mobile device and takes two columns on larger screens:

import flet as ft

ft.ResponsiveRow([
    ft.Column(col={"sm": 6}, controls=ft.Text("Column 1")),
    ft.Column(col={"sm": 6}, controls=ft.Text("Column 2"))
])

Here is more elaborate example of responsive layout:

import flet as ft

def main(page: ft.Page):
    def page_resize(e):
        pw.value = f"{page.width} px"
        pw.update()

    page.on_resize = page_resize

    pw = ft.Text(bottom=50, right=50, style="displaySmall")
    page.overlay.append(pw)
    page.add(
        ft.ResponsiveRow(
            [
                ft.Container(
                    ft.Text("Column 1"),
                    padding=5,
                    bgcolor=ft.colors.YELLOW,
                    col={"sm": 6, "md": 4, "xl": 2},
                ),
                ft.Container(
                    ft.Text("Column 2"),
                    padding=5,
                    bgcolor=ft.colors.GREEN,
                    col={"sm": 6, "md": 4, "xl": 2},
                ),
                ft.Container(
                    ft.Text("Column 3"),
                    padding=5,
                    bgcolor=ft.colors.BLUE,
                    col={"sm": 6, "md": 4, "xl": 2},
                ),
                ft.Container(
                    ft.Text("Column 4"),
                    padding=5,
                    bgcolor=ft.colors.PINK_300,
                    col={"sm": 6, "md": 4, "xl": 2},
                ),
            ],
        ),
        ft.ResponsiveRow(
            [
                ft.TextField(label="TextField 1", col={"md": 4}),
                ft.TextField(label="TextField 2", col={"md": 4}),
                ft.TextField(label="TextField 3", col={"md": 4}),
            ],
            run_spacing={"xs": 10},
        ),
    )
    page_resize(None)

ft.app(target=main)

ResponsiveRow docs, example.

Other new controls​

This release adds new visual and non-visual controls requested by Flet community and also required to build UI of the upcoming Flet Studio.

BottomSheet​

Shows a modal Material Design bottom sheet:

BottomSheet docs, example.

NavigationBar​

Bottom Navigation bar which offers a persistent and convenient way to switch between primary destinations in an app:

NavigationBar docs, example.

Tooltip​

A tooltip control:

Tooltip docs, example.

HapticFeedback​

Allows access to the haptic feedback (clicks and vibrates) interface on the device.

HapticFeedback docs.

ShakeDetector​

A control to detect phone shakes. Based on shake widget.

ShakeDetector docs.

Other improvements​

Markdown code syntax highlight​

Sample code.

Variable fonts support​

Flutter has finally supported variable fonts and we bring that into Flet too!

Sample code.

Upgrade Flet module to the latest version (pip install flet --upgrade) and let us know what you think!

Enjoy!