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Create Calculator app in Python with Flet

In this tutorial we will show you, step-by-step, how to create a Calculator app in Python using Flet framework and deploy it as a web app. The app is a simple console program, yet it is a multi-platform application with similar to iPhone calculator app UI:

You can find the live demo here.

In this tutorial, we will cover all of the basic concepts for creating a web app: building a page layout, adding controls, making reusable UI components, handling events, and deployment options.

The tutorial consists of the following steps:

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:

hello.py
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

Now you are ready to create a calculator app.

To start, you'll need a Text control for showing the result of calculation, and a few ElevatedButtons with all the numbers and actions on them.

Create calc.py with the following contents:

calc.py
import flet
from flet import ElevatedButton, Page, Text


def main(page: Page):
page.title = "Calc App"
result = Text(value="0")

page.add(
result,
ElevatedButton(text="AC"),
ElevatedButton(text="+/-"),
ElevatedButton(text="%"),
ElevatedButton(text="/"),
ElevatedButton(text="7"),
ElevatedButton(text="8"),
ElevatedButton(text="9"),
ElevatedButton(text="*"),
ElevatedButton(text="4"),
ElevatedButton(text="5"),
ElevatedButton(text="6"),
ElevatedButton(text="-"),
ElevatedButton(text="1"),
ElevatedButton(text="2"),
ElevatedButton(text="3"),
ElevatedButton(text="+"),
ElevatedButton(text="0"),
ElevatedButton(text="."),
ElevatedButton(text="="),
)


flet.app(target=main)

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

Building page layout

Now let's arrange the text and buttons in 6 horizontal rows.

Replace calc.py contents with the following:

calc.py
import flet
from flet import ElevatedButton, Page, Row, Text


def main(page: Page):
page.title = "Calc App"
result = Text(value="0")

page.add(
Row(controls=[result]),
Row(
controls=[
ElevatedButton(text="AC"),
ElevatedButton(text="+/-"),
ElevatedButton(text="%"),
ElevatedButton(text="/"),
]
),
Row(
controls=[
ElevatedButton(text="7"),
ElevatedButton(text="8"),
ElevatedButton(text="9"),
ElevatedButton(text="*"),
]
),
Row(
controls=[
ElevatedButton(text="4"),
ElevatedButton(text="5"),
ElevatedButton(text="6"),
ElevatedButton(text="-"),
]
),
Row(
controls=[
ElevatedButton(text="1"),
ElevatedButton(text="2"),
ElevatedButton(text="3"),
ElevatedButton(text="+"),
]
),
Row(
controls=[
ElevatedButton(text="0"),
ElevatedButton(text="."),
ElevatedButton(text="="),
]
),
)


flet.app(target=main)

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

Using Container for decoration

To add a black background with rounded border around the calculator, we will be using Container control. Container may decorate only one control, so we will need to wrap all the 6 rows into a single vertical Column that will be used as the container's content:

To complete the UI portion of the program, update color and size properties for the Text, and color and bgcolor properties for the buttons. For even alignment of the buttons within the rows, we will be using expand property as shown on the diagram above.

Since the program is too long now to be fully included in this tutorial, copy the entire code for this step from here. Run the app and you should see a page like this:

Just what we wanted!

Reusable UI components

While you can continue writing your 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 (for example, at Flet we will be using this Calculator app in a bigger "Gallery" app that will show all the examples for Flet framework).

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 Calc app component, we are going to encapsulate its state and presentation logic in a separate CalculatorApp class. Copy the entire code for this step from here.

Read more about creating user controls.

Try something

Try adding two CalculatorApp components to the page:

# create application instance
calc1 = CalculatorApp()
calc2 = CalculatorApp()

# add application's root control to the page
page.add(calc1, calc2)

Handling events

Now let's make the calculator do its job. We will be using the same event handler for all the buttons and use data property to differenciate between the actions depending on the button clicked. For each ElevatedButton control, specify on_click=self.button_clicked event and set data property equal to button's text, for example:

ElevatedButton(
text="AC",
bgcolor=colors.BLUE_GREY_100,
color=colors.BLACK,
expand=1,
on_click=self.button_clicked,
data="AC",
)

Below is on_click event handler that will reset the Text value when "AC" button is clicked:

def button_clicked(self, e):
if e.data == "AC":
self.result.value = "0"

With similar approach, specify on_click event and data property for each button and add expected action to the button_clicked event handler depending on e.data value. Copy the entire code for this step from here.

Run the app and see it in the action:

Packaging as a desktop app

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

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 Calculator app into an executable.

Deploying a web app

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

Summary

In this tutorial you have learned how to:

  • Create a simple Flet app;
  • Work with Reusable UI components;
  • Design UI layout using Column, Row and Container controls;
  • Handle events;
  • Package your Flet app into an executable;
  • Deploy your Flet app to the web;

For further reading you can explore controls and examples repository.

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