ScreenManager root 运行良好，除了一行代码

Question

我尝试通过做一个测验应用程序来学习 python 和 kivy。 到目前为止，一切对我来说都很好。 登录应用程序后，我想选择有多少玩家在游戏中（例如，我按下有 20 名玩家的按钮）我想被重定向到带有问题和答案选项的屏幕（问题是显示的图像在屏幕上）但我收到此错误

~ raise ScreenManagerException('No Screen with name "%s".' % name) kivy.uix.screenmanager.ScreenManagerException: No Screen with name "start". ~

我多次使用 app.root.current 并且每次都工作到现在，我无法弄清楚出了什么问题..

谢谢 ！！

ALL PY CODE

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import ObjectProperty
from kivy.uix.popup import Popup
from kivy.uix.label import Label
from database import DataBase
import random


class CreateAccountWindow(Screen):
    namee = ObjectProperty(None)
    email = ObjectProperty(None)
    password = ObjectProperty(None)

    def submit(self):
        if self.namee.text != "" and self.email.text != "" and self.email.text.count("@") == 1 and self.email.text.count(".") > 0:
            if self.password != "":
                db.add_user(self.email.text, self.password.text, self.namee.text)

                self.reset()

                sm.current = "login"
            else:
                invalidForm()
        else:
            invalidForm()

    def login(self):
        self.reset()
        sm.current = "login"

    def reset(self):
        self.email.text = ""
        self.password.text = ""
        self.namee.text = ""


class LoginWindow(Screen):
    email = ObjectProperty(None)
    password = ObjectProperty(None)

    def loginBtn(self):
        if db.validate(self.email.text, self.password.text):
            MainWindow.current = self.email.text
            self.reset()
            sm.current = "main"
        else:
            invalidLogin()

    def createBtn(self):
        self.reset()
        sm.current = "create"

    def reset(self):
        self.email.text = ""
        self.password.text = ""


class MainWindow(Screen):
    n = ObjectProperty(None)
    created = ObjectProperty(None)
    email = ObjectProperty(None)
    current = ""

    def logOut(self):
        sm.current = "login"


class Q1(Screen):
    pass


class Q2(Screen):
    pass


class Q3(Screen):
    pass


class Q4(Screen):
    pass


class WrongAnswer(Screen):
    pass


class TestApp:

    def wrong_answer(self):
        screen = ['WrongAnswer']
        return random.choice(screen)

    def random_screen(self):
        screens = ['Q1', 'Q2', 'Q3', 'Q4']
        return random.choice(screens)

    def build(self):

        sm = ScreenManager()
        sm.add_widget(Q1(name='Q1'))
        sm.add_widget(Q2(name='Q2'))
        sm.add_widget(Q3(name='Q3'))
        sm.add_widget(Q4(name='Q4'))
        sm.add_widget(WrongAnswer(name='WrongAnswer'))

        return sm



class WindowManager(ScreenManager):
    pass


def invalidLogin():
    pop = Popup(title='Invalid Login',
                  content=Label(text='Invalid username or password.'),
                  size_hint=(None, None), size=(400, 400))
    pop.open()


def invalidForm():
    pop = Popup(title='Invalid Form',
                  content=Label(text='Please fill in all inputs with valid information.'),
                  size_hint=(None, None), size=(400, 400))

    pop.open()


kv = Builder.load_file("my.kv")

sm = WindowManager()
db = DataBase("users.txt")

screens = [LoginWindow(name="login"), CreateAccountWindow(name="create"),MainWindow(name="main")]
for screen in screens:
    sm.add_widget(screen)

sm.current = "login"


class MyMainApp(App):
    def build(self):
        return sm


if __name__ == "__main__":
    MyMainApp().run()

ALL KV CODE


<CreateAccountWindow>:
    name: "create"

    namee: namee
    email: email
    password: passw

    FloatLayout:
        cols:1

        FloatLayout:
            size: root.width, root.height/2

            Label:
                text: "Create an Account"
                size_hint: 0.8, 0.2
                pos_hint: {"x":0.16, "top":1}
                font_size: (root.width**2 + root.height**2) / 14**4

            Label:
                size_hint: 0.5,0.12
                pos_hint: {"x":0, "top":0.8}
                text: "Name: "
                font_size: (root.width**2 + root.height**2) / 14**4

            TextInput:
                pos_hint: {"x": 0.38, "top":0.77}
                size_hint: 0.4, 0.075
                id: namee
                multiline: False
                font_size: (root.width**2 + root.height**2) / 14**4

            Label:
                size_hint: 0.5,0.12
                pos_hint: {"x":0, "top":0.8-0.13}
                text: "E-Mail: "
                font_size: (root.width**2 + root.height**2) / 14**4

            TextInput:
                pos_hint: {"x": 0.38, "top":0.65}
                size_hint: 0.4, 0.075
                id: email
                multiline: False
                font_size: (root.width**2 + root.height**2) / 14**4

            Label:
                size_hint: 0.5,0.12
                pos_hint: {"x":0, "top":0.8-0.125*2}
                text: "Password: "
                font_size: (root.width**2 + root.height**2) / 14**4

            TextInput:
                pos_hint: {"x": 0.38, "top":0.53}
                size_hint: 0.4, 0.075
                id: passw
                multiline: False
                password: True
                font_size: (root.width**2 + root.height**2) / 14**4

        Button:
            pos_hint:{"x":0.60,"y":0.35}
            size_hint: 0.18, 0.075
            font_size: (root.width**2.11 + root.height**2) / 17**4
            text: "Back to Login"
            on_release:
                root.manager.transition.direction = "left"
                root.login()

        Button:
            pos_hint:{"x":0.38,"y":0.35}
            size_hint: 0.18, 0.075
            text: "Register"
            font_size: (root.width**1.95 + root.height**2) / 14**4
            on_release:
                root.manager.transition.direction = "left"
                root.submit()


<LoginWindow>:
    name: "login"

    email: email
    password: password

    FloatLayout:

        Label:
            text:"E-Mail: "
            font_size: (root.width**1.8 + root.height**2) / 13**4
            pos_hint: {"x":0.1, "top":0.9}
            size_hint: 0.40, 0.15

        TextInput:
            id: email
            font_size: (root.width**1.91 + root.height**2) / 13**4
            multiline: False
            pos_hint: {"x": 0.38 , "top":0.86}
            size_hint: 0.4, 0.075

        Label:
            text:"Password: "
            font_size: (root.width**1.8 + root.height**2) / 13**4
            pos_hint: {"x":0.106, "top":0.81}
            size_hint: 0.35, 0.15

        TextInput:
            id: password
            font_size: (root.width**1.91 + root.height**2) / 13**4
            multiline: False
            password: True
            pos_hint: {"x": 0.38, "top":0.77}
            size_hint: 0.4, 0.075

        Button:
            pos_hint:{"x":0.38,"y":0.605}
            size_hint: 0.18, 0.075
            font_size: (root.width**1.8 + root.height**2) / 13**4
            text: "Login"
            on_release:
                root.manager.transition.direction = "up"
                root.loginBtn()

        Button:
            pos_hint:{"x":0.60,"y":0.605}
            size_hint: 0.18, 0.075
            font_size: (root.width**2.09 + root.height**2) / 17**4
            text: "Create Account"
            on_release:
                root.manager.transition.direction = "right"
                root.createBtn()


<MainWindow>:

    FloatLayout:
        Button:
            pos_hint:{"x": 0.6, "top":0.9}
            size_hint:0.4, 0.1
            text: "5 Players"

        Button:
            pos_hint:{"x": 0.6, "top":0.7}
            size_hint:0.4, 0.1
            text: "10 Players"

        Button:
            pos_hint:{"x": 0.6, "top":0.5}
            size_hint:0.4, 0.1
            text: "15 Players"

        Button:
            pos_hint:{"x": 0.6, "top":0.3}
            size_hint:0.4, 0.1
            text: "20 Players"
            on_release:
                app.root.current = "start"
                root.manager.transition.direction = "down"

        Button:
            pos_hint:{"x":0.04, "y": 0.05}
            size_hint:0.2,0.1
            text: "Log Out"
            on_release:
                app.root.current = "login"
                root.manager.transition.direction = "down"

<Q1>:
    name: "start"
    Image:
        source: 'Q1.png'

        FloatLayout:
            size: root.width, root.height/2

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.09, "top":1.16}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.random_screen()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.5, "top":1.16}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.5, "top":0.7}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.09, "top":0.7}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()


<Q2>:
    Image:
        source: 'Q2.png'

        FloatLayout:
            size: root.width, root.height/2

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.09, "top":1.16}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.5, "top":1.16}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.random_screen()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.5, "top":0.7}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.09, "top":0.7}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

<Q3>:
    Image:
        source: 'Q3.png'

        FloatLayout:
            size: root.width, root.height/2

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.09, "top":1.16}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.5, "top":1.16}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.5, "top":0.7}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.09, "top":0.7}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.random_screen()

<Q4>:
    Image:
        source: 'Q4.png'

        FloatLayout:
            size: root.width, root.height/2

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.09, "top":1.16}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.random_screen()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.5, "top":1.16}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.5, "top":0.7}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

            Button:
                size_hint: 0.3, 0.25
                pos_hint: {"x":0.09, "top":0.7}
                background_color: 1, 1, 1, 0.2
                on_release: root.manager.current = app.wrong_answer()

<WrongAnswer>:
    Image:
        source: 'L1.png'

Answer 1

您没有创建start Screen ：

screens = [LoginWindow(name="login"), CreateAccountWindow(name="create"),MainWindow(name="main")]

我看到login 、 create和main 。