条件分岐付き画面切替のやり方(Python/Kivy)

[andy19ka]

勉強会での鈴木くんのアドバイスのもと
kvファイル上でscreen毎に分けて
pyファイル上で条件分岐をしようと思ったのですが
上手くいきませんでした。

プログラムの一部を載せます。
[pyファイル]
Subject = 0
Price = 0

class ForSmile1(FloatLayout):
class Screen2(Screen):
def on_command (self, command):
if command == 'list':
if Subject != 0 and Price == 0:
sm = ScreenManager()
sm.add_widget(ForSmile1(name='screen3'))
sm.add_widget(ForSmile1(name='screen2'))
elif command == 'Price':
if Subject == 0 and Price != 0:
sm = ScreenManager()
sm.switch_to(screen2)

presentation = Builder.load_file("ForSmile2.kv")

[kvファイル]
:
AnchorLayout:
anchor_x: 'center'
anchor_y: 'bottom'
ScreenManager:
size_hint: 1, .9
id: _screen_manager
:
Label:
markup: True
text: '[size=24]Welcome to [color=4169e1]ForSmile[/color][/size]'

        <Screen2>:
            BoxLayout:
                orientation: 'vertical'
                spacing: 10
                BoxLayout:
                    orientation: 'vertical'
                    Label:
                        markup: True
                        text: '[size=20][color=dd88ff]List[/color][/size]'
                        color: 1, 0, 1, 1
                        size:(400, 20)
                    Button:
                        text: "2015.12.1 ******"
                        on_press: root.on_command('list')
                        font_size: 10
                        background_color: (1, 1, 1, 1)
                        size: (20,10)
                    Button:
                        text: "2016.4.1 ******"
                        on_press: root.on_command('list')
                        font_size: 10
                        background_color: (1, 1, 1, 1)
                        size: (20,10)
        <Screen3>:
            BoxLayout:
                orientation: 'vertical'
                spacing: 10
                BoxLayout:
                    orientation: 'vertical'
                    Label:
                        markup: True
                        text: '[size=20][color=228b22]Donation[/color][/size]'
                        background_color: (1, 0 , 1, 1)
                        size:(400, 20)
                    Button:
                        text: "100yen"
                        on_press: Price = 100, root.on_command('Price')
                        font_size: 10
                        background_color: (1, 1, 1, 1)
                        size: (20,10)
                    Button:
                        text: "210yen"
                        on_press: Price = 210, root.on_command('Price')
                        font_size: 10
                        background_color: (1, 1, 1, 1)
                        size: (20,10)
                    Button:
                        text: "500yen"
                        on_press: Price = 500, root.on_command('Price')
                        font_size: 10
                        background_color: (1, 1, 1, 1)
                        size: (20,10)
                    Button:
                        text: "410yen"
                        on_press: Price = 410, root.on_command('Price')
                        font_size: 10
                        background_color: (1, 1, 1, 1)
                        size: (20,10)
                    Button:
                        text: "2000yen"
                        on_press: Price = 2000, root.on_command('Price')
                        font_size: 10
                        background_color: (1, 1, 1, 1)
                        size: (20,10)
                    Button:
                        text: "3000yen"
                        on_press: Price = 3000, root.on_command('Price')
                        font_size: 10
                        background_color: (1, 1, 1, 1)
                        size: (20,10)

[andy19ka]

Gistにあげました！
よろしくお願いします！！
[main.py]
https://gist.github.com/andy19ka/ea7a4a97ef6ad4e48363

[ForSmile2.kv]
https://gist.github.com/andy19ka/f64859411189c36f0513