geekdoc-python-zh/docs/pythonguru/020.md

Python 运算符重载

原文： https://thepythonguru.com/python-operator-overloading/

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于 2020 年 1 月 7 日更新

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您已经看到可以使用+运算符添加数字，并同时连接字符串。 这是可能的，因为int类和str类都重载了+运算符。 运算符实际上是在各个类中定义的方法。 运算符的定义方法称为运算符重载。 例如：要对自定义对象使用+运算符，您需要定义一个名为__add__的方法。

让我们举个例子来更好地理解

import math

class Circle:

    def __init__(self, radius):
        self.__radius = radius

    def setRadius(self, radius):
        self.__radius = radius

    def getRadius(self):
        return self.__radius

    def area(self):
        return math.pi * self.__radius ** 2

    def __add__(self, another_circle):
        return Circle( self.__radius + another_circle.__radius )

c1 = Circle(4)
print(c1.getRadius())

c2 = Circle(5)
print(c2.getRadius())

c3 = c1 + c2 # This became possible because we have overloaded + operator by adding a    method named __add__
print(c3.getRadius())

预期输出：

4
5
9

import math

class Circle:

    def __init__(self, radius):
        self.__radius = radius

    def setRadius(self, radius):
        self.__radius = radius

    def getRadius(self):
        return self.__radius

    def area(self):
        return math.pi * self.__radius ** 2

    def __add__(self, another_circle):
        return Circle( self.__radius + another_circle.__radius )

c1 = Circle(4)
print(c1.getRadius())

c2 = Circle(5)
print(c2.getRadius())

c3 = c1 + c2 # This became possible because we have overloaded + operator by adding a    method named __add__
print(c3.getRadius()) 

在上面的示例中，我们添加了__add__()方法，该方法允许使用+运算符添加两个圆形对象。 在__add__()方法内部，我们正在创建一个新对象并将其返回给调用者。

Python 还有许多其他特殊方法，例如__add__()，请参见下面的列表。

运算符	函数	方法说明
+	__add__(self, other)	加法
*	__mul__(self, other)	乘法
-	__sub__(self, other)	减法
%	__mod__(self, other)	余数
/	__truediv__(self, other)	除法
<	__lt__(self, other)	小于
<=	__le__(self, other)	小于或等于
==	__eq__(self, other)	等于
!=	__ne__(self, other)	不等于
>	__gt__(self, other)	大于

>=，__ge__(self, other)，大于或等于[index]，__getitem__(self, index)，索引运算符in，__contains__(self, value)，检查成员资格len，__len__(self)，元素数str，__str__(self)的字符串表示形式

下面的程序使用上面提到的一些函数来重载运算符。

import math

class Circle:

    def __init__(self, radius):
        self.__radius = radius

    def setRadius(self, radius):
        self.__radius = radius

    def getRadius(self):
        return self.__radius

    def area(self):
        return math.pi * self.__radius ** 2

    def __add__(self, another_circle):
        return Circle( self.__radius + another_circle.__radius )

    def __gt__(self, another_circle):
        return self.__radius > another_circle.__radius

    def __lt__(self, another_circle):
        return self.__radius < another_circle.__radius

    def __str__(self):
        return "Circle with radius " + str(self.__radius)

c1 = Circle(4)
print(c1.getRadius())

c2 = Circle(5)
print(c2.getRadius())

c3 = c1 + c2
print(c3.getRadius())

print( c3 > c2) # Became possible because we have added __gt__ method

print( c1 < c2) # Became possible because we have added __lt__ method

print(c3) # Became possible because we have added __str__ method

预期输出：

4
5
9
True
True
Circle with radius 9

import math

class Circle:

    def __init__(self, radius):
        self.__radius = radius

    def setRadius(self, radius):
        self.__radius = radius

    def getRadius(self):
        return self.__radius

    def area(self):
        return math.pi * self.__radius ** 2

    def __add__(self, another_circle):
        return Circle( self.__radius + another_circle.__radius )

    def __gt__(self, another_circle):
        return self.__radius > another_circle.__radius

    def __lt__(self, another_circle):
        return self.__radius < another_circle.__radius

    def __str__(self):
        return "Circle with radius " + str(self.__radius)

c1 = Circle(4)
print(c1.getRadius())

c2 = Circle(5)
print(c2.getRadius())

c3 = c1 + c2
print(c3.getRadius())

print( c3 > c2) # Became possible because we have added __gt__ method

print( c1 < c2) # Became possible because we have added __lt__ method

print(c3) # Became possible because we have added __str__ method 

下一课是继承和多态。

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