Kivy开发中点击单词按钮触发的Label无法正常显示如何解决

• 已实现基础功能：从预设列表读取句子，将句子转为按钮排布在嵌套于FloatLayout的BoxLayout中，支持自定义句子显示位置；点击句子按钮后，原按钮移除，替换为句子拆分出的多个独立单词按钮。
• 待实现需求：在BoxLayout下方添加Label控件，点击任意单词按钮时，Label立刻显示与被点击按钮完全一致的文本。例：点击句子按钮| Hello World, this is an example sentence. |后拆分出多个单词按钮，点击sentence按钮时，下方Label同步显示sentence。
• 现存问题：运行代码后点击单词按钮，Label无任何内容显示，原代码如下：

import kivy

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Widget, Label
from kivy.config import Config

sentences = ['This is example sentence 1.', 'This is example sentence 2.', 'This is example sentence 3.', 'This is example sentence 4.', 'This is example sentence 5.', 'This is example sentence 6.']

words = [ ['This', 'is' , 'example' ,  'sentence' , '1' ], ['This', 'is' , 'example' ,  'sentence' , '2' ], ['This', 'is' , 'example' ,  'sentence' , '3' ], ['This', 'is' , 'example' ,  'sentence' , '4' ], ['This', 'is' , 'example' ,  'sentence' , '5' ], ['This', 'is' , 'example' ,  'sentence' , '6' ] ]



class AdaptiveButton(Button):
    """自适应尺寸按钮，大小匹配文本内容"""
    def __init__(self, **kwargs):
        super(AdaptiveButton, self).__init__(**kwargs)
        self.size_hint = (None, None)
        self.bind(texture_size = self.setter("size"))


class Boxlayout(BoxLayout):
    """排布所有交互控件"""
    def __init__(self, **kwargs):
        super(Boxlayout, self).__init__(**kwargs)
        self.orientation = "vertical"
        self.spacing = "10dp"

        for i, line in enumerate(sentences):
            abtn = AdaptiveButton(text = line, font_size=30) 
            abtn.bind(on_press = lambda btn, j = i : self.destroy_then_create(btn, j))
            self.add_widget(abtn)
    def destroy_then_create(self, btn, index):
        self.remove_widget(btn)

        box = BoxLayout(size_hint = (None, None), spacing = "10dp")
        box.bind(minimum_size = box.setter("size"))
        sentence = words[index]
        i = len(sentences)-index-1
        for word in sentence:
            abtn = AdaptiveButton(text = word)
            abtn.bind(on_press=lambda btn, j=i: show(word)) # 绑定标签显示逻辑
            box.add_widget(abtn)

        self.add_widget(box, i)

class Meaningclass(Label): # 标签类
    pass


def show(k): # 标签更新函数
    MeC = Meaningclass()
    Meaning = Label(text=k, font_size=30)
    MeC.add_widget(Meaning)


class MyApp(App):
    def build(self):
        self.icon = 'budspencer.png'
        Fl = FloatLayout()
        Box = Boxlayout(pos_hint={'x': .23, 'y': .58}) 
        meaning = Meaningclass(pos_hint={'x': .9, 'y': .1}) 
        Fl.add_widget(Box)
        Fl.add_widget(meaning) # 将标签添加到FloatLayout
        return Fl


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

1. 完全没操作界面上已存在的Label实例：show函数每次触发都会新建一个没挂载到任何布局上的Meaningclass和内部Label，这些控件根本不在界面渲染树里，不可能显示出来。
2. 存在闭包陷阱：单词按钮绑定事件时用的word是循环变量，循环结束后所有按钮的回调拿到的word都是最后一个单词的值，和按钮实际文本不匹配。
3. 冗余嵌套：Meaningclass本身已经继承了Label，不需要再往里面嵌套一个新的Label控件，属于多余写法。
4. 引用断裂：单词按钮的回调逻辑拿不到已经添加到FloatLayout里的那个meaning标签实例，没法修改它的文本属性。

修改逻辑：

• 删掉全局无效的show函数，给Boxlayout增加参数接收目标Label的实例引用，存在类属性里供回调调用。
• 修复闭包问题，点击单词按钮时直接取按钮自身的text属性赋值给Label，完全避免循环变量取值错误。
• 简化Meaningclass，初始化时直接设置好字体大小属性，去掉冗余的子Label嵌套。

import kivy
from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label

sentences = ['This is example sentence 1.', 'This is example sentence 2.', 'This is example sentence 3.', 'This is example sentence 4.', 'This is example sentence 5.', 'This is example sentence 6.']
words = [ ['This', 'is' , 'example' ,  'sentence' , '1' ], ['This', 'is' , 'example' ,  'sentence' , '2' ], ['This', 'is' , 'example' ,  'sentence' , '3' ], ['This', 'is' , 'example' ,  'sentence' , '4' ], ['This', 'is' , 'example' ,  'sentence' , '5' ], ['This', 'is' , 'example' ,  'sentence' , '6' ] ]


class AdaptiveButton(Button):
    """自适应尺寸按钮，大小匹配文本内容"""
    def __init__(self, **kwargs):
        super(AdaptiveButton, self).__init__(**kwargs)
        self.size_hint = (None, None)
        self.bind(texture_size = self.setter("size"))


class Boxlayout(BoxLayout):
    """排布所有交互控件，接收外部传入的目标显示标签引用"""
    def __init__(self, meaning_label, **kwargs):
        super(Boxlayout, self).__init__(**kwargs)
        self.orientation = "vertical"
        self.spacing = "10dp"
        # 存下要更新的Label实例
        self.meaning_label = meaning_label

        for i, line in enumerate(sentences):
            abtn = AdaptiveButton(text = line, font_size=30) 
            abtn.bind(on_press = lambda btn, j = i : self.destroy_then_create(btn, j))
            self.add_widget(abtn)
    
    def destroy_then_create(self, btn, index):
        self.remove_widget(btn)
        box = BoxLayout(size_hint = (None, None), spacing = "10dp")
        box.bind(minimum_size = box.setter("size"))
        sentence = words[index]
        insert_pos = len(sentences)-index-1
        for word in sentence:
            abtn = AdaptiveButton(text=word, font_size=30)
            # 直接取按钮自身的text赋值给目标Label，避免闭包问题
            abtn.bind(on_press=lambda clicked_btn: setattr(self.meaning_label, 'text', clicked_btn.text))
            box.add_widget(abtn)
        self.add_widget(box, insert_pos)


class Meaningclass(Label):
    """显示单词文本的标签"""
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.font_size = 30


class MyApp(App):
    def build(self):
        self.icon = 'budspencer.png'
        Fl = FloatLayout()
        # 先创建Label实例
        meaning = Meaningclass(pos_hint={'x': .5, 'y': .1})
        # 把Label实例传给Boxlayout，供按钮回调更新
        Box = Boxlayout(meaning_label=meaning, pos_hint={'x': .23, 'y': .58}) 
        Fl.add_widget(Box)
        Fl.add_widget(meaning)
        return Fl


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

内容的提问来源于stack exchange，提问作者Delicious_pie