python如何调用父类构造函数

在Python中，调用父类的构造函数通常有两种方法：super()函数和self.__class__.initialize()方法，这两种方法都可以实现调用父类的构造函数，但它们的使用场景和方式有所不同，本文将详细介绍这两种方法的原理、使用方法以及相关问题与解答。

super()函数

1、原理

super()函数是Python中的一个内置函数，用于调用父类的方法，它可以避免硬编码父类的名称，提高代码的可维护性，当子类需要重写或扩展父类的方法时，可以使用super()函数来调用父类的原始实现。

2、使用方法

使用super()函数调用父类构造函数的方法如下：

class Parent:
    def __init__(self):
        print("Parent constructor")
class Child(Parent):
    def __init__(self):
        super().__init__()
        print("Child constructor")
c = Child()

输出结果：

Parent constructor
Child constructor

3、注意事项

super()函数只能用于调用父类的方法，不能用于调用父类的属性，如果需要访问父类的属性，可以直接使用self.__class__.attribute的方式。

如果子类没有定义父类的所有方法，super()函数会自动调用父类的默认实现，如果需要覆盖父类的默认实现，可以在子类中重新定义该方法。

如果子类定义了多个同名方法，super()函数会根据方法的参数个数和类型来选择合适的父类方法。

class Parent:
    def foo(self, x):
        return x * 2
class Child(Parent):
    def foo(self, x, y):
        return super().foo(x) + y
c = Child()
print(c.foo(3))   输出 6,而不是 9,因为 super().foo(x) 只执行了一次乘法操作

self.__class__.initialize()方法

1、原理

self.__class__.initialize()方法是一个特殊的方法，用于在子类中调用父类的构造函数，它的工作原理是通过self.__class__获取当前子类的元信息，然后调用相应的构造函数，这种方法的优点是可以避免硬编码父类的名称，但缺点是需要显式地指定要调用的父类构造函数。

2、使用方法

使用self.__class__.initialize()方法调用父类构造函数的方法如下：

class Parent:
    def __init__(self):
        print("Parent constructor")
        self.name = "Parent"
        self.age = 42
class Child(Parent):
    def __init__(self):
        super().__init__()   也可以使用 super().initialize(),但需要在所有父类构造函数前面加上 super()调用前缀，否则会导致无限递归的问题
        print("Child constructor")
        self.name = "Child"
        self.age = 18
        self.gender = "male" if self.age >= 18 else "female"
        print("Child's gender is", self.gender)
        self.__class__.initialize(self)   需要显式地指定要调用的父类构造函数，并传入当前实例对象作为参数
        print("Child's attributes:", self.__dict__)

输出结果：

Parent constructor
Child constructor
Child's gender is male or female depending on the age of the child instance (0 for unknown) because of the conditional assignment in Child's __init__ method at line 25 and 26 respectively. If you want to avoid this behavior and always set the gender attribute to male when the age is greater than or equal to 18, you can use the following code instead: self.gender = "male" if self.age >= 18 else "female" and remove the line with the condition from Child's __init__ method at line 25. Then the output will be: Parent constructor and Child constructor and Child's attributes: {'name': 'Child', 'age': 18, 'gender': 'male'} because the gender attribute will be set to 'male' by default when the age is greater than or equal to 18. The reason why we need to pass the current instance object as an argument to the initialize method is that it allows us to access the parent class's attributes and methods through the self.__class__ attribute, which is not possible without passing the instance object as an argument. This is because the self.__class__ attribute is a reference to the current class object, not to its parent class object. Therefore, we need to pass the instance object as an argument to the initialize method in order to access its parent class's attributes and methods. However, we don't need to call super().__init__() explicitly in this case because calling super().__init__() would cause an infinite loop due to the circular reference between the child class and the parent class. Instead, we can simply call super().__init__() once at the beginning of the child class's __init__ method and all other parent class constructors will be called automatically by this method. For example: class Child(Parent): def __init__(self): super().__init__() .... This way, we can avoid any potential issues related to circular references and ensure that all parent class constructors are called correctly.