How to implement nested classes in Python

Q: Can classes be written in python classes?

A: Yes, classes in Python can contain other classes. These classes are called nested classes or nested types. The syntax for defining a nested class within a class is the same as for defining a class within a module. For example, the following is an example that contains nested classes:

class OuterClass:
    def __init__(self):
        self.inner = self.InnerClass()

    class InnerClass:
        def __init__(self):
            self.inner_var = 42

outer = OuterClass()
print(outer.inner.inner_var) # 输出：42

In this example, OuterClass contains a nested class InnerClass, and an instance of InnerClass is initialized in the constructor of OuterClass. The member variable inner_var in InnerClass can be accessed by accessing outer.inner.inner_var.

It should be noted that there are no special access restrictions between nested classes and their external classes. They can access each other's member variables and methods. Additionally, nested classes can also be static or dynamic (i.e. they can contain instance methods or not).

Q: How does InnerClass access member variables of external classes?

A: In Python, nested classes can access member variables of external classes through external class name.member variable name. For example, in the previous example, if you want InnerClass to access the member variable outer_var of OuterClass, you can modify the code as follows:

class OuterClass:
    def __init__(self):
        self.outer_var = 100
        self.inner = self.InnerClass()

    class InnerClass:
        def __init__(self):
            outer_var = OuterClass.outer_var
            print(outer_var)

outer = OuterClass() # 输出：100

In this example, the constructor of InnerClass passes OuterClass.outer_var The method accesses the member variable outer_var in OuterClass. It should be noted that when accessing member variables of an external class, you need to use the name of the external class, not self, because the nested class and the external class are two different classes.

Q: For example, if A nests B, how to create a new B object in B's function?

A: If class B is defined in class A, then in Creating a new B object in B's function can be achieved in the following way:

class A:
    class B:
        def __init__(self, val):
            self.val = val

        def create_new_b(self, val):
            new_b = A.B(val)
            return new_b

a = A()
b = a.B(1)
new_b = b.create_new_b(2)
print(b.val)  # 输出 1
print(new_b.val)  # 输出 2

In this example, A contains a nested class B, and B's constructor init receives a parameter val. Class B also contains a method named create_new_b, which receives a parameter val and creates a new class B object using A.B(val). The create_new_b method returns the newly created B object. A new class B object can be created by calling the b.create_new_b(2) method, where b is a class B instance within a class A object.

It should be noted that when creating a class B object, you need to use A.B instead of B. This is because B is nested in A, not in the global namespace.

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