kivy语言：if

from kivy.config import Config
Config.set('graphics','width','400')
Config.set('graphics', 'width', '300')

from kivy.lang import Builder
from kivy.uix.widget import Widget
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import BooleanProperty, NumericProperty
from kivy.clock import Clock
from kivy.app import App


Builder.load_string('''
#:kivy 1.8.0

<KivyTimer>:
    BoxLayout:
        orientation: 'vertical'
        pos: root.pos
        size: root.size

        Label:
            text: str(root.left_time)
            font_size: 100

        BoxLayout:
            orientation: 'horizontal'
            size_hint: 1.0, 0.3

            Button:
                text: '+1 minute'
                font_size: 16
                on_press: root.on_command('+1 minute')

            Button:
                text: 'Stop' if root.is_countdown else 'Start'
                font_size: 16
                on_press: root.on_command('start/stop')

            Button:
                text: 'Reset'
                font_size: 16
                on_press: root.on_command('reset')
''')



class KivyTimer(Widget):
    is_countdown = BooleanProperty(False)
    left_time = NumericProperty(0)

    def on_command(self, command):
        if command == '+1 minute':
            self.left_time += 60
        elif command == 'start/stop':
            if self.is_countdown:
                self.stop_timer()
            elif self.left_time > 0:
                self.start_timer()
        elif command == 'reset':
            self.stop_timer()
            self.left_time = 0

    def on_countdown(self, dt):
        self.left_time -= 1
        if self.left_time == 0:
            self.is_countdown = False
            return False

    def start_timer(self):
        self.is_countdown = False
        Clock.schedule_interval(self.on_countdown, 1.0)

    def stop_timer(self):
        self.is_countdown = False

        Clock.unschedule(self.on_countdown)


class KivyTimerApp(App):
    def build(self):
        return KivyTimer()



if __name__ == '__main__':
    KivyTimerApp().run()