Flet

Posted on Jun 30, 2022

Create 2-in-1 web and desktop app in Python without knowledge of HTML/CSS/JavaScript

#webdev #python #beginners #tutorial

In this tutorial we will show you, step-by-step, how to create a ToDo web app in Python using Flet framework and then share it on the internet. The app is a single-file console program of just 180 lines (formatted!) of Python code, yet it is a multi-session, modern single-page application with rich, responsive UI:

You can see the live demo here.

We chose a ToDo app for the tutorial, because it covers all of the basic concepts you would need to create any web app: building a page layout, adding controls, handling events, displaying and editing lists, making reusable UI components, and deployment/packaging options.

The tutorial consists of the following steps:

Getting started with Flet
Adding page controls and handling events
View, edit and delete list items
Filtering list items
Final touches
Deploying a web app
Packaging as a desktop app

Getting started with Flet

To write a Flet web app you don't need to know HTML, CSS or JavaScript, but you do need a basic knowledge of Python and object-oriented programming.

Flet requires Python 3.7 or above. To create a web app in Python with Flet, you need to install flet module first:

pip install flet

To start, let's create a simple hello-world app.

Create hello.py with the following contents:

import flet
from flet import Page, Text

def main(page: Page):
    page.add(Text(value="Hello, world!"))

flet.app(target=main)

Run this app and you will see a new window with a greeting:

Adding page controls and handling events

Now we're ready to create a multi-user ToDo app.

To start, we'll need a TextField for entering a task name, and an "+" FloatingActionButton with an event handler that will display a Checkbox with a new task.

Create todo.py with the following contents:

import flet
from flet import Checkbox, FloatingActionButton, Page, TextField, icons

def main(page: Page):
    def add_clicked(e):
        page.add(Checkbox(label=new_task.value))
        new_task.value = ""
        page.update()

    new_task = TextField(hint_text="Whats needs to be done?")

    page.add(new_task, FloatingActionButton(icon=icons.ADD, on_click=add_clicked))

flet.app(target=main)

Run the app and you should see a page like this:

Page layout

Now let's make the app look nice! We want the entire app to be at the top center of the page, taking up 600 px width. The TextField and the "+" button should be aligned horizontally, and take up full app width:

Row is a control that is used to lay its children controls out horizontally on a page. Column is a control that is used to lay its children controls out vertically on a page.

Replace todo.py contents with the following:

import flet
from flet import Checkbox, Column, FloatingActionButton, Page, Row, TextField, icons


def main(page: Page):
    def add_clicked(e):
        tasks_view.controls.append(Checkbox(label=new_task.value))
        new_task.value = ""
        view.update()

    new_task = TextField(hint_text="Whats needs to be done?", expand=True)
    tasks_view = Column()
    view = Column(
        width=600,
        controls=[
            Row(
                controls=[
                    new_task,
                    FloatingActionButton(icon=icons.ADD, on_click=add_clicked),
                ],
            ),
            tasks_view,
        ],
    )

    page.horizontal_alignment = "center"
    page.add(view)

flet.app(target=main)

Run the app and you should see a page like this:

Reusable UI components

While we could continue writing our app in the main function, the best practice would be to create a reusable UI component. Imagine you are working on an app header, a side menu, or UI that will be a part of a larger project. Even if you can't think of such uses right now, we still recommend creating all your web apps with composability and reusability in mind.

To make a reusable ToDo app component, we are going to encapsulate its state and presentation logic in a separate class:

import flet
from flet import Checkbox, Column, FloatingActionButton, Page, Row, TextField, UserControl, icons

class TodoApp(UserControl):
    def build(self):
        self.new_task = TextField(hint_text="Whats needs to be done?", expand=True)
        self.tasks = Column()

        # application's root control (i.e. "view") containing all other controls
        return Column(
            width=600,
            controls=[
                Row(
                    controls=[
                        self.new_task,
                        FloatingActionButton(icon=icons.ADD, on_click=self.add_clicked),
                    ],
                ),
                self.tasks,
            ],
        )

    def add_clicked(self, e):
        self.tasks.controls.append(Checkbox(label=self.new_task.value))
        self.new_task.value = ""
        self.update()


def main(page: Page):
    page.title = "ToDo App"
    page.horizontal_alignment = "center"
    page.update()

    # create application instance
    todo = TodoApp()

    # add application's root control to the page
    page.add(todo)

flet.app(target=main)

Read more about [creating user controls](https://flet.dev/docs/getting-started/python#user-controls).

Try something:
Try adding two TodoApp components to the page:

# create application instance
app1 = TodoApp()
app2 = TodoApp()

# add application's root control to the page
page.add(app1, app2)

View, edit and delete list items

In the previous step, we created a basic ToDo app with task items shown as checkboxes. Let's improve the app by adding "Edit" and "Delete" buttons next to a task name. The "Edit" button will switch a task item to edit mode.

Each task item is represented by two rows: display_view row with Checkbox, "Edit" and "Delete" buttons and edit_view row with TextField and "Save" button. view column serves as a container for both display_view and edit_view rows.

Before this step, the code was short enough to be fully included in the tutorial. Going forward, we will be highlighting only the changes introduced in a step.

Copy the entire code for this step from here. Below we will explain the changes we've done to implement view, edit, and delete tasks.

To encapsulate task item views and actions, we introduced a new Task class:

class Task(UserControl):
    def __init__(self, task_name):
        super().__init__()
        self.task_name = task_name

    def build(self):
        self.display_task = Checkbox(value=False, label=self.task_name)
        self.edit_name = TextField(expand=1)

        self.display_view = Row(
            alignment="spaceBetween",
            vertical_alignment="center",
            controls=[
                self.display_task,
                Row(
                    spacing=0,
                    controls=[
                        IconButton(
                            icon=icons.CREATE_OUTLINED,
                            tooltip="Edit To-Do",
                            on_click=self.edit_clicked,
                        ),
                        IconButton(
                            icons.DELETE_OUTLINE,
                            tooltip="Delete To-Do",
                            on_click=self.delete_clicked,
                        ),
                    ],
                ),
            ],
        )

        self.edit_view = Row(
            visible=False,
            alignment="spaceBetween",
            vertical_alignment="center",
            controls=[
                self.edit_name,
                IconButton(
                    icon=icons.DONE_OUTLINE_OUTLINED,
                    icon_color=colors.GREEN,
                    tooltip="Update To-Do",
                    on_click=self.save_clicked,
                ),
            ],
        )
        return Column(controls=[self.display_view, self.edit_view])

    def edit_clicked(self, e):
        self.edit_name.value = self.display_task.label
        self.display_view.visible = False
        self.edit_view.visible = True
        self.update()

    def save_clicked(self, e):
        self.display_task.label = self.edit_name.value
        self.display_view.visible = True
        self.edit_view.visible = False
        self.update()

Additionally, we changed TodoApp class to create and hold Task instances when the "Add" button is clicked:

class TodoApp(UserControl):
    def build(self):
        self.new_task = TextField(hint_text="Whats needs to be done?", expand=True)
        self.tasks = Column()
        # ...

    def add_clicked(self, e):
        task = Task(self.new_task.value, self.task_delete)
        self.tasks.controls.append(task)
        self.new_task.value = ""
        self.update()

For "Delete" task operation, we implemented task_delete() method in TodoApp class which accepts task control instance as a parameter:

class TodoApp(UserControl):
    # ...
    def task_delete(self, task):
        self.tasks.controls.remove(task)
        self.update()

Then, we passed a reference to task_delete method into Task constructor and called it on "Delete" button event handler:

class Task(UserControl):
    def __init__(self, task_name, task_delete):
        super().__init__()
        self.task_name = task_name
        self.task_delete = task_delete

        # ...        

    def delete_clicked(self, e):
        self.task_delete(self)

Run the app and try to edit and delete tasks:

Filtering list items

We already have a functional ToDo app where we can create, edit, and delete tasks. To be even more productive, we want to be able to filter tasks by their status.

Copy the entire code for this step from here. Below we will explain the changes we've done to implement filtering.

Tabs control is used to display filter:

from flet import Tabs, Tab

# ...

class TodoApp(UserControl):
    def __init__(self):
        self.tasks = []
        self.new_task = TextField(hint_text="Whats needs to be done?", expand=True)
        self.tasks = Column()

        self.filter = Tabs(
            selected_index=0,
            on_change=self.tabs_changed,
            tabs=[Tab(text="all"), Tab(text="active"), Tab(text="completed")],
        )

        self.view = Column(
            width=600,
            controls=[
                Row(
                    controls=[
                        self.new_task,
                        FloatingActionButton(icon=icons.ADD, on_click=self.add_clicked),
                    ],
                ),
                Column(
                    spacing=25,
                    controls=[
                        self.filter,
                        self.tasks,
                    ],
                ),
            ],
        )

To display different lists of tasks depending on their statuses, we could maintain three lists with "All", "Active" and "Completed" tasks. We, however, chose an easier approach where we maintain the same list and only change a task's visibility depending on its status.

In TodoApp class we overrided update() method which iterates through all the tasks and updates their visible property depending on the status of the task:

class TodoApp(UserControl):

    # ...

    def update(self):
        status = self.filter.tabs[self.filter.selected_index].text
        for task in self.tasks.controls:
            task.visible = (
                status == "all"
                or (status == "active" and task.completed == False)
                or (status == "completed" and task.completed)
            )
        super().update()

Filtering should occur when we click on a tab or change a task status. TodoApp.update() method is called when Tabs selected value is changed or Task item checkbox is clicked:

class TodoApp(UserControl):

    # ...

    def tabs_changed(self, e):
        self.update()

class Task(UserControl):
    def __init__(self, task_name, task_status_change, task_delete):
        super().__init__()
        self.completed = False
        self.task_name = task_name
        self.task_status_change = task_status_change
        self.task_delete = task_delete

    def build(self):
        self.display_task = Checkbox(
            value=False, label=self.task_name, on_change=self.status_changed
        )
        # ...

    def status_changed(self, e):
        self.completed = self.display_task.value
        self.task_status_change(self)

Run the app and try filtering tasks by clicking on the tabs:

Final touches

Our Todo app is almost complete now. As a final touch, we will add a footer (Column control) displaying the number of incomplete tasks (Text control) and a "Clear completed" button.

Copy the entire code for this step from here. Below we highlighted the changes we've done to implement the footer:

class TodoApp():
    def __init__(self):
        # ...

        self.items_left = Text("0 items left")

        self.view = Column(
            width=600,
            controls=[
                Row([Text(value="Todos", style="headlineMedium")], alignment="center"),
                Row(
                    controls=[
                        self.new_task,
                        FloatingActionButton(icon=icons.ADD, on_click=self.add_clicked),
                    ],
                ),
                Column(
                    spacing=25,
                    controls=[
                        self.filter,
                        self.tasks,
                        Row(
                            alignment="spaceBetween",
                            vertical_alignment="center",
                            controls=[
                                self.items_left,
                                OutlinedButton(
                                    text="Clear completed", on_click=self.clear_clicked
                                ),
                            ],
                        ),
                    ],
                ),
            ],
        )

    # ...

    def clear_clicked(self, e):
        for task in self.tasks.controls[:]:
            if task.completed:
                self.task_delete(task)

    def update(self):
        status = self.filter.tabs[self.filter.selected_index].text
        count = 0
        for task in self.tasks.controls:
            task.visible = (
                status == "all"
                or (status == "active" and task.completed == False)
                or (status == "completed" and task.completed)
            )
            if not task.completed:
                count += 1
        self.items_left.value = f"{count} active item(s) left"
        super().update()

Run the app:

Deploying a web app

Congratulations! You have created your first Python app with Flet, and it looks awesome! Now it's time to share your app with the world!

Follow these instructions to deploy your Flet app as a web app to Fly.io or Replit.

Packaging as a desktop app

Flet Python app and all its dependencies can be packaged into an executable and user can run it on their computer without installing a Python interpreter or any modules.

Follow these instructions to package your app into an executable.

Summary

In this tutorial you have learned how to:

Create a simple Flet app;
Work with Reusable UI components;
Design UI layout using Column and Row controls;
Work with lists: view, edit and delete items, filtering;
Deploy your Flet app to the web;
Package your Flet app into an executable;

For further reading you can explore controls and examples repository.

We would love to hear your feedback! Please drop us an email, join the discussion on Discord, follow on Twitter.

Top comments (1)

stasserov • Apr 1 '23

We are waiting for the opportunity to create applications for android :)

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