In part 3 of my Kivy tutorial series we’ll create a working calculator app in one video. We’ll cover how to use layouts inside of other layouts, event handling, TextInput, executing code based on widget events, and much more.

All of the heavily commented code used follows the video below. Feel free to do whatever you’d like with it.

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Code & Cheat Sheet

# ---------- main.py  ---------- 

import kivy
kivy.require("1.9.0")

from kivy.app import App
from kivy.uix.gridlayout import GridLayout

class CalcGridLayout(GridLayout):

    # Function called when equals is pressed
    def calculate(self, calculation):
        if calculation:
            try:
                # Solve formula and display it in entry
                # which is pointed at by display
                self.display.text = str(eval(calculation))
            except Exception:
                self.display.text = "Error"

class CalculatorApp(App):

    def build(self):
        return CalcGridLayout()

calcApp = CalculatorApp()
calcApp.run()

# ---------- calculator.kv  ----------

# Custom button
<CustButton@Button>:
    font_size: 32

# Define id so I can refer to the CalcGridLayout
# class functions
# Display points to the entry widget
<CalcGridLayout>:
    id: calculator
    display: entry
    rows: 5
    padding: 10
    spacing: 10

    # Where input is displayed
    BoxLayout:
        TextInput:
            id: entry
            font_size: 32
            multiline: False

    # When buttons are pressed update the entry
    BoxLayout:
        spacing: 10
        CustButton:
            text: "7"
            on_press: entry.text += self.text
        CustButton:
            text: "8"
            on_press: entry.text += self.text
        CustButton:
            text: "9"
            on_press: entry.text += self.text
        CustButton:
            text: "+"
            on_press: entry.text += self.text

    BoxLayout:
        spacing: 10
        CustButton:
            text: "4"
            on_press: entry.text += self.text
        CustButton:
            text: "5"
            on_press: entry.text += self.text
        CustButton:
            text: "6"
            on_press: entry.text += self.text
        CustButton:
            text: "-"
            on_press: entry.text += self.text

    BoxLayout:
        spacing: 10
        CustButton:
            text: "1"
            on_press: entry.text += self.text
        CustButton:
            text: "2"
            on_press: entry.text += self.text
        CustButton:
            text: "3"
            on_press: entry.text += self.text
        CustButton:
            text: "*"
            on_press: entry.text += self.text

    # When equals is pressed pass text in the entry
    # to the calculate function
    BoxLayout:
        spacing: 10
        CustButton:
            text: "AC"
            on_press: entry.text = ""
        CustButton:
            text: "0"
            on_press: entry.text += self.text
        CustButton:
            text: "="
            on_press: calculator.calculate(entry.text)
        CustButton:
            text: "/"
            on_press: entry.text += self.text