高级编程技巧 学习笔记
一、让类支持比较操作
有时我们希望自定义类的实例间可以使用,<、<=、>、>=、==、!= 符号进行比较,我们自定义比较的类,例如,有一个矩形的类,比较两个矩形的实例时,比较的是他们的面积。
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int 类型比较的是 大小
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str 类型比较的是 ask 码(一个一个比较)
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集合类型比较的是 包含关系
1.1、在类中定义 __lt__ 和 __eq__
方法
import math
class Rect():
def __init__(self, w, h):
self.w = w
self.h = h
def area(self):
return self.w * self.h
# lt 是小于, ge 是大于
def __lt__(self, other):
return self.area() < other.area()
# eq 是等于
def __eq__(self, other):
return self.area() == other.area()
class Cirle():
def __init__(self, r):
self.r = r
def area(self):
return self.r ** 2*math.pi
def __lt__(self, other):
return self.area() < other.area()
def __eq__(self, other):
return self.area() == other.area()
r1 = Rect(1, 2)
c1 = Cirle(2)
print(r1 < c1)
print(r1 <= c1) # 报错
存在问题,不能进行 >=, ==, <=
的比较。
1.2、引入库 functools
import math
from functools import total_ordering
@total_ordering
class Rect():
def __init__(self, w, h):
self.w = w
self.h = h
def area(self):
return self.w * self.h
# lt 是小于, ge 是大于
def __lt__(self, other):
return self.area() < other.area()
# eq 是等于
def __eq__(self, other):
return self.area() == other.area()
class Cirle():
def __init__(self, r):
self.r = r
def area(self):
return self.r ** 2*math.pi
def __lt__(self, other):
return self.area() < other.area()
def __eq__(self, other):
return self.area() == other.area()
r1 = Rect(1, 2)
r2 = Rect(2, 1)
c1 = Cirle(2)
print(r1 < c1)
print(r1 <= c1)
print(r1 == r2)
存在问题,代码冗余。
1.3、使用 abc 模块,创建抽象基类
import math
from functools import total_ordering
import abc
@total_ordering
class Shape(metaclass=abc.ABCMeta):
@abc.abstractmethod
def area(self, key):
pass
def __lt__(self, other):
return self.area() < other.area()
def __eq__(self, other):
return self.area() == other.area()
class Rect(Shape):
def __init__(self, w, h):
self.w = w
self.h = h
def area(self):
return self.w * self.h
class Cirle(Shape):
def __init__(self, r):
self.r = r
def area(self):
return self.r ** 2*math.pi
r1 = Rect(1, 2)
r2 = Rect(2, 1)
c1 = Cirle(2)
print(r1 < c1)
print(r1 >= c1)
print(r1 == r2)