面向对象：Python的类和Java有什么不一样？

你熟悉的OOP，Python全都有

Java程序员最熟悉的就是面向对象了。Python同样支持OOP，但语法更灵活，限制更少。

没有interface？没有private？没有构造函数重载？Python用另一套方式解决了这些问题。

一、类的定义

# Python
class Student:
    def __init__(self, name, age, score):
        self.name = name
        self.age = age
        self.score = score
    
    def is_pass(self):
        return self.score >= 60
    
    def __str__(self):
        return f"Student({self.name}, {self.age}岁, {self.score}分)"
s = Student("张三", 20, 85)
print(s)          # Student(张三, 20岁, 85分)
print(s.is_pass()) # True

// Java
public class Student {
    private String name;
    private int age;
    private int score;
    
    public Student(String name, int age, int score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }
    
    public boolean isPass() {
        return this.score >= 60;
    }
}

区别：

• init是Python的初始化方法
• self相当于Java的this，但必须显式写
• str相当于toString()
• Python不需要写访问修饰符

二、访问控制：没有private？

Python用命名约定实现：

class BankAccount:
    def __init__(self, owner, balance):
        self.owner = owner      # 公开
        self._balance = balance # 约定protected
        self.__pin = "1234"     # 名称改写private
    
    def get_balance(self):
        return self._balance
account = BankAccount("张三", 10000)
print(account.owner)          # OK
print(account._balance)       # 能访问但不该
print(account.__pin)        # 报错

print(account._BankAccount__pin)  # 能绕过去

哲学： "We are all consenting adults here"。信任程序员，不做强制限制。

三、没有interface？用ABC

from abc import ABC, abstractmethod
class Shape(ABC):
    @abstractmethod
    def area(self):
        pass
  
    @abstractmethod
    def perimeter(self):
        pass
  
    def describe(self):
        return f"面积: {self.area()}, 周长: {self.perimeter()}"
class Circle(Shape):
    def init(self, radius):
        self.radius = radius
  
    def area(self):
        return 3.14159  self.radius * 2
  
    def perimeter(self):
        return 2  3.14159  self.radius
shapes = [Circle(5), Circle(3)]
for shape in shapes:
    print(shape.describe())
s = Shape()  # 报错！不能实例化抽象类

四、@property：Python的getter/setter

class Circle:
    def __init__(self, radius):
        self._radius = radius
    
    @property
    def radius(self):
        return self._radius
    
    @radius.setter
    def radius(self, value):
        if value < 0:
            raise ValueError("半径不能为负数")
        self._radius = value
    
    @property
    def area(self):
        return 3.14159 * self._radius ** 2
c = Circle(5)
print(c.radius)    # 5
c.radius = 10
print(c.area)      # 314.159

五、多重继承

class Flyable:
    def fly(self):
        return "我在飞"
class Swimmable:
    def swim(self):
        return "我在游泳"
class Duck(Flyable, Swimmable):
    pass
duck = Duck()
print(duck.fly())   # "我在飞"
print(duck.swim())  # "我在游泳"

六、魔术方法

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    
    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)
    
    def __eq__(self, other):
        return self.x == other.x and self.y == other.y
    
    def __repr__(self):
        return f"Vector({self.x}, {self.y})"
    
    def __getitem__(self, index):
        if index == 0: return self.x
        if index == 1: return self.y
        raise IndexError
v1 = Vector(1, 2)
v2 = Vector(3, 4)
print(v1 + v2)   # Vector(4, 6)
print(v1 * 3)    # Vector(3, 6)
print(v1[0])     # 1

总结

本系列持续更新中，关注不迷路。