Python重写类时触发NameError：InterfaceWithNoMenu未定义

我基于Kivy框架开发Python GUI应用，尝试修改Settings模块的外观、样式与行为。编写了嵌套类结构的代码，但创建Settings窗口实例时，SettingsWithNoMenu类的__init__方法中引用InterfaceWithNoMenu时触发错误：

self.interface_cls = InterfaceWithNoMenu
NameError: name 'InterfaceWithNoMenu' is not defined

InterfaceWithNoMenu类就在SettingsWithNoMenu类的下方定义，却找不到它，特此求助。

代码示例 #

class Settings(Screen):

    class SettingsWithNoMenu(kivy.uix.settings.SettingsWithNoMenu):

        def __init__(self, *args, **kwargs):
            self.interface_cls = InterfaceWithNoMenu
            kivy.uix.settings.SettingsWithNoMenu.__init__( self, *args, **kwargs )

    class InterfaceWithNoMenu(kivy.uix.settings.ContentPanel):
        def add_widget(self, widget):
            if self.container is not None and len(self.container.children) > 0:
                raise Exception(
                'ContentNoMenu cannot accept more than one settings panel')
            super(InterfaceWithNoMenu, self).add_widget(widget)
            kivy.uix.settings.InterfaceWithNoMenu.__init__( self, *args, **kwargs )

    actionview = ObjectProperty(None)
    settings_content = ObjectProperty(None)

    def __init__(self, **kwargs):
        super(Settings, self).__init__(**kwargs)
        ...

问题原因 #

Python解析类定义是按从上到下的顺序执行的。当SettingsWithNoMenu的__init__方法引用InterfaceWithNoMenu时，这个类还未被定义（它在SettingsWithNoMenu之后才声明），因此触发NameError。另外，嵌套类的作用域属于外层类，直接跨顺序引用会超出当前解析范围。

解决方法 #

方法1：调整嵌套类定义顺序

把InterfaceWithNoMenu移到SettingsWithNoMenu之前定义，确保解析到SettingsWithNoMenu时，目标类已经存在：

class Settings(Screen):

    class InterfaceWithNoMenu(kivy.uix.settings.ContentPanel):
        def __init__(self, *args, **kwargs):
            super().__init__(*args, **kwargs)
            
        def add_widget(self, widget):
            if self.container is not None and len(self.container.children) > 0:
                raise Exception('ContentNoMenu cannot accept more than one settings panel')
            super().add_widget(widget)

    class SettingsWithNoMenu(kivy.uix.settings.SettingsWithNoMenu):

        def __init__(self, *args, **kwargs):
            self.interface_cls = InterfaceWithNoMenu
            kivy.uix.settings.SettingsWithNoMenu.__init__( self, *args, **kwargs )

    actionview = ObjectProperty(None)
    settings_content = ObjectProperty(None)

    def __init__(self, **kwargs):
        super(Settings, self).__init__(**kwargs)
        ...

方法2：通过外层类引用内部类

如果不想调整顺序，可以通过外层Settings类来指定InterfaceWithNoMenu，利用外层类的属性访问内部嵌套类：

class Settings(Screen):

    class SettingsWithNoMenu(kivy.uix.settings.SettingsWithNoMenu):

        def __init__(self, *args, **kwargs):
            # 通过外层类定位内部类
            self.interface_cls = Settings.InterfaceWithNoMenu
            kivy.uix.settings.SettingsWithNoMenu.__init__( self, *args, **kwargs )

    class InterfaceWithNoMenu(kivy.uix.settings.ContentPanel):
        def __init__(self, *args, **kwargs):
            super().__init__(*args, **kwargs)
            
        def add_widget(self, widget):
            if self.container is not None and len(self.container.children) > 0:
                raise Exception('ContentNoMenu cannot accept more than one settings panel')
            super().add_widget(widget)

    actionview = ObjectProperty(None)
    settings_content = ObjectProperty(None)

    def __init__(self, **kwargs):
        super(Settings, self).__init__(**kwargs)
        ...

额外修正说明 #

原代码中InterfaceWithNoMenu类存在两个问题：

1. add_widget方法中调用了kivy.uix.settings.InterfaceWithNoMenu.__init__，但你的类继承的是ContentPanel，且方法未声明*args, **kwargs参数，会导致参数错误，建议移除该调用或修正继承关系。
2. 调用父类方法时可以使用简化的super()语法，无需显式指定类名和self参数。

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