The Magic method in Python

1. What is the Magic method?

The Magic method is a special way to add magic to your class, and if your object implements (overloads) one of these methods, then this method is called by Python in a special case, and you can define the behavior you want, and all of this happens automatically. They are often named after two underscores (such as __init__,__lt__), and Python's magical methods are very powerful, so it's important to know how to use them!

2. __init__ (self[, ...]), __new__ (cls[, ...]), __del__ (self)

The __init__ constructor, which initializes the method when an instance is created. But it is not the first method to instantiate the call, __new__ is the first method that instantiates the object invocation, it only takes down the CLS parameter and passes the other arguments to __init__. __new__ is seldom used, but it also has a suitable scenario, especially when a class inherits from a type that does not change frequently, such as a tuple or a string.

Note the following four points when using __new__:

1. __new__ is the first method 2 that is called when an object is instantiated. Its first parameter is this class, and the other parameters are used to pass directly to the __init__ Method 3. __new__ decides whether to use the __init__ method, because __new__ can invoke the constructor of another class or return the other instance object directly as an instance of this class, and if __new__ does not return an instance object, __init__ will not be called 4. __NEW__ is primarily used to inherit an immutable type such as a tuple or string
5. __new__ return is an example of a build

__new__ Implementing a singleton pattern:

class Person (object):    def __init__ (self, Name, age):        self.name = name        Self.age = Age    def __new__ (CLS, * args, **kwargs):        If not hasattr (CLS, ' instance '):            cls.instance = Super (PERSON,CLS). __new__ (CLS)        return Cls.instancea = person (' P1 ', "b") = Person (' p2 ', +) print (a = = b) #这里的打印结果是True, visible A, and B are the same instance # Singleton role: #第一, control the use of resources, Through thread synchronization to control the concurrent access of resources, #第二, control the number of instances generated, to achieve the purpose of saving resources. #第三, used as a communication medium, that is, data sharing, it can not establish a direct association under the conditions, so that more than the # #相关的两个线程或者进程之间实现通信. #比如, the design of database connection pool is generally based on singleton mode, and database connection is a kind of database resource.

__del__ destructor, called when an instance is destroyed

3. __call__ (Self[,args ...]), __getitem__ (Self,key), __setitem__ (Self,key,value)

__CALL__ allows an instance of a class to be called like a function

class Person (object):    def __init__ (self, Name, age):        self.name = name        Self.age = Age        self.instance = add    def __call__ (Self,*args):        return Self.instance (*args) def add (args):    return args[0] + args[1]a = person (' P1 ', ') print (A ([)]) #这里将打印 3 #可见当创建a这个对象之后, if the __call__ function is defined, the object can be called as a function.

__getitem__ definition Gets the behavior of the specified element in the container, equivalent to Self[key]

class Person (object):    def __init__ (self, Name, age):        self.name = name        Self.age = Age        self._registry = { c4/> ' name ': Name,            ' age ': Age        }    def __call__ (self, *args):        return Self.instance (*args)    def __  Getitem__ (self, key):        If key isn't in Self._registry.keys ():            raise Exception (' Please registry the key:%s first! ') % (key,))        return self._registry[key]a = person (' P1 ', ' ") Print (a[' name '],a[' age ') #这里打印的是 ' P1 ' #可见__getitem__ Make an instance accessible like a dictionary

__setitem__ setting the behavior of the specified element in the container, equivalent to self[key] = value

4, __getattr__ (Self,name), __getattribute__ (Self,name), __setattr__ (Self,name,value), __delattr__ (Self,name), __get__ (Self,instance,owner), __set__ (Self,instance,value), __delete__ (self,instance)

__GETATTR__ defines the behavior when a user attempts to access a property that does not exist

__SETATTR__ defines the behavior when a property is set

__GETATTRIBUTE__ defines the behavior when a property is accessed

　　

class Person (object):    def __init__ (self, Name, age):        self.name = name        Self.age = Age        self._registry = { c4/> ' name ': Name,            ' age ': ' Age '    def __getattr__ (self, item):        print ("T has the attribute", item)        return False    def __setattr__ (self, Key, value):        Self.__dict__[key] = value    def __getattribute__ ( Self, item):        #注意此处不要用 Self.__dict__[item]        #因为self. __dict__ will still be intercepted by __getattribute__ so that it will fall into the loop        return. OBJECT.__GETATTRIBUTE__ (self,item) a = person (' p1 ', ') print (A.HH)        #这里会打印 don ' t has the attribute hh and falsea.hh = ' FDF '        #这里设置该属性值为 ' fdf ' Print (a.hh)           #这里将打印出 FDF

__DELATTR__ defines the behavior when a property is deleted

__GET__ defines the behavior when a descriptor's value is obtained

__SET__ defines the behavior when a descriptor's value is set

__DELETE__ defines the behavior when the value of the descriptor is deleted

Class Descriptor (object):    def __init__ (self):        self.des = None    def __get__ (self, instance, owner):        Return Instance.__dict__.get (self.des,none)    def __set__ (self, instance, value):        instance.__dict__[self.des ] = Valueclass Person (object):    des = descriptor () #这里的Descriptor就是一个描述符类    def __init__ (self, Name, age):        Self.name = name        Self.age = Age        self._registry = {            ' name ': Name,            ' age ': Age        }a = person (' p1 ', 20) A.des = ten    #这里会调用Descriptor的__set__方法print (a.des)   #这里会调用Descriptor的__get__方法

About the descriptor have doubts friends can own Baidu also can refer to this article http://www.geekfan.net/7862/, there are many related articles on the Internet, here is not the focus of the descriptor so there is no explanation for too many descriptors!

　　

　　

　　

The Magic method in Python