calc.kv

BoxLayout:
    orientation: 'vertical'
    Label:
        size_hint_y: None
        height: '50dp'
        id: calc
        text: ''

    GridLayout:
        cols: 4
        Button:
            text: '+'
            on_press: calc.text += '+'
        Button:
            text: '-'
            on_press: calc.text += '-'
        Button:
            text: '*'
            on_press: calc.text += '*'
        Button:
            text: '/'
            on_press: calc.text += '/'
        Button:
            text: '9'
            on_press: calc.text += '9'
        Button:
            text: '8'
            on_press: calc.text += '8'
        Button:
            text: '7'
            on_press: calc.text += '7'
        Button:
            text: 'CE'
            on_press: calc.text = ''
        Button:
            text: '6'
            on_press: calc.text += '6'
        Button:
            text: '5'
            on_press: calc.text += '5'
        Button:
            text: '4'
            on_press: calc.text += '4'
        # let's rowspan 2
        Widget:
            Button:
                y: placeholder1.y
                x: self.parent.x
                width: self.parent.width
                height: self.parent.height + placeholder1.height
                text: '='
                on_press: app.calc(calc)
        Button:
            text: '3'
            on_press: calc.text += '3'
        Button:
            text: '2'
            on_press: calc.text += '2'
        Button:
            text: '1'
            on_press: calc.text += '1'
        Widget:
            id: placeholder1
            # placeholder for the '=' rowspan

        # let's colspan 4
        Widget:
            Button:
                pos: self.parent.pos
                height: self.parent.height
                width: self.parent.width * 4
                text: '0'
                on_press: calc.text += '0'

test_rowspan.py

from kivy.app import App


class CalcApp(App):
    def calc(self, label):
        try:
            label.text = str(eval(label.text))
        except:
            label.text = 'syn error'

CalcApp().run()