Kivy嵌套TaskTemplate时报AttributeError属性不存在错误

我定义了包含Button、TextInput等控件的TaskTemplate组件，核心代码片段如下：

class TaskTemplate(BoxLayout, Button):
    task_name = StringProperty()
    def __init__(self):
        super(TaskTemplate, self).__init__()
        self.update_state = 'down'

对应的kv规则：

<TaskTemplate>:
    RelativeLayout:
        CheckBox:
            state: root.update_state

• 当TaskTemplate作为应用根类使用时，程序运行完全正常，可以通过update_state参数修改CheckBox的选中状态。
• 但将TaskTemplate按如下结构添加到MainScreen中时，程序抛出无限循环报错：

MainScreen:
<MainScreen>:
    BoxLayout:
        ScrollView:
            GridLayout:
                id: here_add_task_template

报错核心信息为AttributeError: 'TaskTemplate' object has no attribute 'update_state'，完整报错栈如下：

Traceback (most recent call last):
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\venv\lib\site-packages\kivy\lang\builder.py", line 240, in create_handler
     return eval(value, idmap), bound_list
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\middleScreen.kv", line 122, in <module>
     state: root.update_state
   File "kivy\weakproxy.pyx", line 32, in kivy.weakproxy.WeakProxy.__getattr__
 AttributeError: 'TaskTemplate' object has no attribute 'update_state'
 
 During handling of the above exception, another exception occurred:
 
 Traceback (most recent call last):
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\venv\lib\site-packages\kivy\lang\builder.py", line 694, in _apply_rule
     value, bound = create_handler(
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\venv\lib\site-packages\kivy\lang\builder.py", line 243, in create_handler
     raise BuilderException(rule.ctx, rule.line,
 kivy.lang.builder.BuilderException: Parser: File "C:\Users\barbu\OneDrive\Desktop\Final App 2\middleScreen.kv", line 122:
 ...
     120:#                root.update_index()
     121:#                root.save_in_json(self)
 >>  122:            state: root.update_state
     123:
     124:        Button:
 ...
 AttributeError: 'TaskTemplate' object has no attribute 'update_state'
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\venv\lib\site-packages\kivy\lang\builder.py", line 240, in create_handler
     return eval(value, idmap), bound_list
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\middleScreen.kv", line 122, in <module>
     state: root.update_state
   File "kivy\weakproxy.pyx", line 32, in kivy.weakproxy.WeakProxy.__getattr__
 
 
 During handling of the above exception, another exception occurred:
 
 Traceback (most recent call last):
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\main.py", line 246, in <module>
     MainApp().run()
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\venv\lib\site-packages\kivy\app.py", line 955, in run
     runTouchApp()
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\venv\lib\site-packages\kivy\base.py", line 574, in runTouchApp
     EventLoop.mainloop()
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\venv\lib\site-packages\kivy\base.py", line 341, in mainloop
     self.window.mainloop()
   File "C:\Users\barbu\OneDrive\Desktop\Final App 2\venv\lib\site-packages\kivy\core\window\window_sdl2.py", line 757, in mainloop
     if self.dispatch('on_key_down', key,
   File "kivy\_event.pyx", line 727, in kivy._event.EventDispatcher.dispatch

报错由两个核心问题触发：

1. 继承顺序错误：Kivy控件多继承时，事件交互类需要放在布局类前面，否则会打乱MRO初始化顺序，导致属性、事件注册异常。你写的class TaskTemplate(BoxLayout, Button)顺序完全写反。
2. 属性定义方式错误：update_state是你在__init__里动态绑定的普通实例属性，不是Kivy原生的Property类型。组件作为根节点时，kv规则解析时机刚好在__init__执行完成后，所以能找到属性；但组件作为子组件嵌套加载时，kv规则解析会发生在__init__属性赋值之前，就会触发属性找不到的错误，再叠加Kivy的规则自动重试机制，就会出现无限循环抛错的现象。

按如下步骤修改即可解决问题：

• 调整类继承顺序，把交互控件类Button放在布局类BoxLayout前面
• 把update_state定义为类级别的StringProperty（CheckBox的state接受字符串值，对应'normal'/'down'两个状态），直接给默认值，不要在__init__里动态赋值普通属性
• 给__init__方法添加**kwargs参数，调用父类初始化时把参数透传，避免Kivy控件初始化时丢失必要参数

修改后的Python代码参考：

from kivy.properties import StringProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

class TaskTemplate(Button, BoxLayout):
    task_name = StringProperty()
    update_state = StringProperty('down')
    def __init__(self, **kwargs):
        super(TaskTemplate, self).__init__(**kwargs)

修改后组件无论作为根节点，还是动态添加到MainScreen的GridLayout中，都能正常加载运行，不会再抛出属性不存在的错误。

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