python魔法方法

1、__new__() //实例化对象调用第一个方法并不是__init__而是__new__，一般Python默认调用,一般只有在继承了不可变的类要去修改里面内容，需要重写
eg:
    class CapStr(str):
        def __new__(cls,string):
        string = string.upper()
        return str.__new__(cls,string)
调用:
cap = CapStr('aa')
print(cap)//AA

2、__init__() //构造方法不能有任何返回
3、__del__() //对象将要被销毁，会自动被调用，跟php中的_destroy ，垃圾回收机制，当所有对象引用都被删除，才会调用
eg:

class YO:

def __init__(self):

print('111')

def __del__(self):

print('22')
调用:
yo = YO()
yo1 = yo
del yo //此时不会调用__del__
del yo1 //两个都删掉才会调用__del__ 22

4、算数运算

4.1 正向运算

class New_int(int):    def __add__(self, other):        return int.__sub__(self,other)    def __sub__(self, other):        return int.__add__(self, other)a = New_int(2)b = New_int(3)print(a+b) # -1print(a-b) # 5

4.2 逆向运算

class New_int(int):    def __radd__(self, other):        return int.__sub__(self,other)a = 3b = New_int(2)print(a+b) #-1，因为3不是New_int，找不到，就会执行b的逆向运算radd，self是b，other是a

 5、__setattr__()、__getattr()__、__getattribute()__、__delattr__()
__setattr__()：对象给属性赋值会调用此方法
__getattribute__()：读取对象属性调用此方法(不管属性是否存在都会调用)
__getattr__()：对象属性不存在才会调用，此方法在__getattribute__()之后调用
__delattr__()：删除一个对象的属性，调用此方法

class MyTime:

    def __setattr__(self,name,val):

        print('setattr')

        return super().__setattr__(name,val)

    def __getattribute__(self,name):

        print('getattribute')

        return super().__getattribute__(name)

    def __getattr__(self,name):

        print('getattr')

    def __delattr__(self,name):

        print('delattr')

        return super().__delattr__(name)

m = MyTime()

m.a

m.a = 2

m.a

del m.a

 
6、描述符__get__ 、__set__、__delete__
实例一：

class MyProperty:

    def __init__(self,fget=None,fset=None,fdel=None):

        self.fget = fget

        self.fset = fset

        self.fdel = fdel

    def __set__(self,instance,val):

        self.fset(instance,val)

    def __get__(self,instance,owener):

        return self.fget(instance)

    def __delete__(self,instance):

        self.fdel(instance)

class C:

    def __init__(self):

        self._x = 0

    def getContent(self):

        return self._x

    def setContent(self,val):

        self._x = val

    def delContent(self):

        del self._x

    x = MyProperty(getContent,setContent,delContent)

c = C()

print(c.x)

c.x = 11

print(c.x)

 
实例二：

class A:

    def __init__(self,value=0):

        self.value = value

    def __set__(self,instance,val):

        self.value = val

    def __get__(self,instance,owener):

        return self.value

class B:

    def __init__(self,value=0):

        self.value = value

    def __set__(self,instance,val):

       instance.a = val + 1

    def __get__(self,instance,owener):

        return   instance.a

class C:

    a = A()

    b = B()

c = C()

print(c.a)

c.a = 11

print(c.a)

c.b = 10

print(c.a)

print(c.b)

 

7、定制序列

class Test:

    def __init__(self,*args):

        self.values = [x for x in args]

        self.count = {}.fromkeys(range(len(self.values)),0)

    def __len__(self):

        return len(self.count)

    def __getitem__(self,key):

        self.count[key] += 1

        return self.values[key]

t = Test(1,2,3,4,5,6)

print(t[1])

print(t[2])

print(t[1])

print(t.count)