Python进阶10---魔术方法*

佚名 6年前 (2019-04-19) 随笔 1459人围观抢沙发百度已收录

特殊属性

SRE实战互联网时代守护先锋，助力企业售后服务体系运筹帷幄！一键直达领取阿里云限量特价优惠。

查看属性

#animal.py
class Animal:
    x = 123
    def __init__(self,name):
        self._name = name
        self.__age = 10
        self.weight = 20

print('animal Module\'s names = {}'.format(dir()))
#animal Module's names = ['Animal', '__annotations__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__']

#cat.py
import animal
from animal import Animal
class Cat(Animal):
    x = 'cat'
    y = 'abcd'

class Dog(Animal):
    def __dir__(self):
        return ['dog']#指定返回列表

print('*'*10)
print('Current Module : {}'.format(dir()))
print('animal Module : {}'.format(dir(animal)))
print('Cat Module : {}'.format(dir(Cat)))
print('object Module : {}'.format(sorted(object.__dict__)))
print('object Module : {}'.format(sorted(object.__dict__.keys())))
print('*'*10)
tom = Cat('tom')
dog = Dog('dog')
print(sorted(dir(tom)))
print(sorted(tom.__dict__))
print(sorted(dir(Dog)))
print(sorted(dir(dog)))
#dir()的等价，近似如下，__dict__字典中几乎包括了所有属性
print(sorted(set(tom.__dict__.keys())|set(Cat.__dict__.keys())|set(object.__dict__.keys())))

魔术方法

hash

class A:
    def __init__(self):
        self.a = 'a'
        self.b = 'b'

    def __hash__(self):
        return 1

print(hash(A()))#1
print(A(),A())#<__main__.A object at 0x00000000028F54E0> <__main__.A object at 0x00000000028F55F8>
a = A()
print({A(),A()})#{<__main__.A object at 0x00000000028F55F8>, <__main__.A object at 0x0000000002993F60>}
s = {a,a}
print(s)#{<__main__.A object at 0x00000000028F54E0>} 
# set内部机制会先调用is判断是否是同一对象的引用

#如果想要实现set提出相同的key，还需要增加__eq__函数

class A:
    def __init__(self):
        self.a = 'a'
        self.b = 'b'

    def __hash__(self):
        return 1

    def __eq__(self, other):
        return self.a == other.a    #相当于是return True

print(hash(A()))#1
print(A(),A())#<__main__.A object at 0x00000000029054E0> <__main__.A object at 0x00000000029055F8>
print({A(),A()})#{<__main__.A object at 0x00000000029055F8>}
s = {A(),A()}
print(s)#{<__main__.A object at 0x00000000029055F8>}

查看源码后发现源码中有一句__hash__=None,也就是如果调用__hash__()相当于None(),一定报错。所有类都继承object,
而这个类是具有__hash__()方法的，如果一个类不能被hash，就是把__hash__设置成None了。

#注意
from collections import Hashable
print(isinstance(list(),Hashable))#False
print(isinstance(list,Hashable))#True

class Point:
    def __init__(self,x,y):
        self.x = x
        self.y = y

    def __hash__(self):
        return hash((self.x,self.y))

    def __eq__(self, other):
        return self.x == other.x and self.y == other.y

p1 = Point(4,5)
p2 = Point(4,5)
print(hash(p1))#3713084879518070856
print(hash(p2))#3713084879518070856
print(p1 is p2)#False
print(p1 == p2)#True
print(set((p1,p2)))#{<__main__.Point object at 0x0000000002993E48>}
print(isinstance(p1,Hashable))#True

bool

class A:
    pass

print(bool(A))#True
print(bool(A()))#True

class B(object):
    def __bool__(self):
        return False

print(bool(B))#True
print(bool(B()))#False

class C:
    def __len__(self):
        return 0

print(bool(C))#True
print(bool(C()))#False

可视化

class A:
    def __init__(self):
        self.a = 'a'
        self.b = 'b'

    def __repr__(self):
        return 'repr:{} {}'.format(self.a,self.b)

    def __str__(self):
        return 'str:{} {}'.format(self.a,self.b)

print(A())#print函数使用__str__
print([A()])#[]使用__str__,但其内部使用__repr__
print(str(A()))#[]使用__str__，str()函数也使用__str__
a1 = A()
a2 = A()
lst = [a1,a2]
print(lst)#[repr:a b, repr:a b]
for i in lst:  #str:a b    str:a b
    print(i)