Calculator.kv

<Custom_Button@Button>:
    font_size: 24

# Define the calculator layout
# Which can be referred to Layout Class

<CalcLayout>:
    id: calculator
    display: entry
    rows: 6
    padding: 10
    spacing: 10

    BoxLayout:
        # When input triggered, updating entry
        TextInput:
            id: entry
            font_size: 24
            multiline: False

    # Buttons of numbers
    # Three numbers and a sign is a box, from up to down

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

        Custom_Button:
            text: "Delete"
            on_press: calculator.delete(entry.text)

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

        Custom_Button:
            text: "Add"
            on_press:entry.text += "+"
    BoxLayout:
        spacing: 10
        Custom_Button:
            text: "7"
            on_press: entry.text += self.text
        Custom_Button:
            text: "8"
            on_press: entry.text += self.text
        Custom_Button:
            text: "9"
            on_press: entry.text += self.text

        Custom_Button:
            text: "Remove"
            on_press: entry.text += "-"

    BoxLayout:
        spacing: 10
        Custom_Button:
            text: "."
            on_press: entry.text += self.text

        Custom_Button:
            text: "0"
            on_press: entry.text += self.text

        Custom_Button:
            text: "*"
            on_press: entry.text += self.text

        Custom_Button:
            text: "Tax"
            on_press: calculator.tax(entry.text)

    BoxLayout:
        Custom_Button:
            font_size: 24
            text: "AC"
            on_press: entry.text = ""

        Custom_Button:
            text: "done"
            on_press: calculator.calculate(entry.text)