Python syntax allows redefining classes within classes, which are called nested classes or inner classes

Python syntax allows classes to be defined within classes, which are called nested classes or inner classes. In Python, classes can be treated as objects, so other classes can be created inside the class's definition.

The definition of a nested class occurs within the scope of the outer class, and the nested class can access the properties and methods of the outer class. Nested classes can define properties, methods, and special methods just like ordinary classes, and can be instantiated and used.

Here is a simple example code that demonstrates how to define nested classes within classes in Python:

class OuterClass:
    outer_var = 10
    
    def outer_method(self):
        print("This is the outer method")
    
    class NestedClass:
        nested_var = 20
        
        def nested_method(self):
            print("This is the nested method")

In the example above, OuterClassthe outer class NestedClassis the nested class. We can use nested classes just like normal classes:

# 创建外部类对象
outer_obj = OuterClass()

# 创建嵌套类对象
nested_obj = OuterClass.NestedClass()

# 访问外部类属性和方法
print(outer_obj.outer_var)           # 输出: 10
outer_obj.outer_method()             # 输出: This is the outer method

# 访问嵌套类属性和方法
print(nested_obj.nested_var)         # 输出: 20
nested_obj.nested_method()           # 输出: This is the nested method

The use of nested classes can provide a better way to organize your code, keeping related classes together, making your code more readable and maintainable. It should be noted that nested classes are not required, but whether to use nested classes is determined according to specific design requirements and code structure.

In addition, special attention should be paid to the fact that nested classes cannot directly use the properties and methods of external classes. If you want to use them, please follow the steps below:

If you want to access the properties of the outer class in the nested class, you can pass the properties of the outer class to the nested class by passing an instance of the outer class as a parameter or by other means. Here is a modified sample code:

class OuterClass:
    outer_var = 10
    
    def outer_method(self):
        print("This is the outer method")
    
    class NestedClass:
        def __init__(self, outer_instance):
            self.outer_instance = outer_instance
        
        def nested_method(self):
            print("This is the nested method")
            print("Outer var:", self.outer_instance.outer_var)

In this example, we pass the instance of the outer class outer_instanceas a parameter to the constructor of the nested class __init__and save it as a property of the nested class self.outer_instance. Then, in the method of the nested class, we can use self.outer_instance.outer_varto access the properties of the outer class.

Now, let's use the modified example code to test:

# 创建外部类对象
outer_obj = OuterClass()

# 创建嵌套类对象，并传递外部类实例
nested_obj = OuterClass.NestedClass(outer_obj)

# 访问嵌套类方法
nested_obj.nested_method()

The output will be:

This is the nested method
Outer var: 10

In this way, we successfully access the properties of the outer class in the nested class. Please note that when using a nested class, it is necessary to decide whether to access the properties of the outer class in the nested class according to the specific requirements and design, and choose an appropriate way to achieve such access.

Regarding the above code:

self.outer_instance = outer_instance

For a detailed explanation, please refer to the blog post:
Why are variables in the constructor of a Python class usually preceded by "self."?