Double underline method in Python class

__getitem__,__setitem__ and __delitem__

　　Implements a dictionary operation of object properties.

classPerson :def __init__(self, name, age, Hobby): Self.name=name Self.age=Age Self.hobby=Hobbydef __getitem__(self, item):ifhasattr (self, item):returnSelf.__dict__[item]def __setitem__(self, Key, value): Self.__dict__[Key] =valuedef __delitem__(self, key):delSelf.__dict__[Key]zxc= Person ('Zxc', 26,'Read')Print(Zxc.name)#zxc Object Native View Properties methodPrint(zxc['name'])#ZXC viewing methods implemented through GetItemzxc['name'] ='Zzy'  #implementing modifications with SetItemzxc['Weight'] = 70#increased by SetItem implementationPrint(ZXC.__dict__)#{' weight ': +, ' name ': ' Zzy ', ' hobby ': ' read ', ' Age ':delzxc['Hobby']#Delete with Delitem implementationPrint(ZXC.__dict__)#{' name ': ' Zzy ', ' weight ': +, ' age ': +}

__new__: 　　 Construction Method: Create an Object

Instantiate to use the __new__ method

 class   Foo:  def  __init__   = name  def  __new__  (CLS, *args, * * kwargs):  return   '   Create an object  "  obj  = Foo ( "   ZXC   ") #   When instantiating an object, the __new__ method is called.  print  (obj) #   print: Create an object  

classFoo:def __init__(self, name): Self.name=namedef __new__(CLS, *args, * *Kwargs):returnObject.__new__(Foo)#the __new__ method inside object is used to construct objectsobj= Foo ('Zxc')Print(obj)#<__main__. Foo object at 0x000002cadd5c0048>

use of the __new__ method: Singleton mode

　　A programming pattern: A class always has only one instance

classFoo:__instance=Falsedef __init__(self, Name, age): Self.name=name Self.age= Agedef __new__(CLS, *args, * *Kwargs):ifCls.__instance:#after instantiating an object, the subsequent instantiation uses the previous object            returnCls.__instanceCLS.__instance= object.__new__(CLS)returnCls.__instancea= Foo ('Zxc', 25) b= Foo ('ZXF', 22)Print(A.__dict__)#{' name ': ' Zxf ', ' Age ': $}Print(b.__dict__)#{' name ': ' Zxf ', ' Age ': $}B.hobby ='Read'Print(A.hobby)#Read#A and B are the same object

　　__eq__ and __hash__

classFoo:def __init__(self, name): Self.name=Namea= Foo ('Zxc') b= Foo ('Zxc')Print(A = = b)#False normal A class of two objects even if the property is the same he's different.classFoo:def __init__(self, name): Self.name=namedef __eq__(self, Other):ifSelf.name = =Other.name:returnTrueElse:            returnFalsea= Foo ('Zxc') b= Foo ('Zxc')Print(a)#<__main__. Foo object at 0x000001543ba60048>Print(b)#<__main__. Foo object at 0x000001543ba604e0>Print(A = = b)#true A and B are not the same, but the result is true, which means that the __eq__ method is called when the = = comparison is used, and the default is to use the __eq__ method of the object.

class Foo:     def __hash__ (self):         return = Foo ()print(hash (a))  #  Ten   The built-in function hash is called the object's __hash__ method .

set will depend on __eq__ and __hash__

classFoo:def __init__(self, name, age, Sex): Self.name=name Self.age=Age Self.sex=Sexdef __hash__(self):returnHash (self.name+self.sex)def __eq__(self, Other):ifSelf.name = = Other.name andSelf.sex = =Other.sex:returnTrueElse:            returnFalsea= Foo ('Zxc', 25,'male') b= Foo ('Zxc', 24,'male')Print(Set ([A, b]))#{<__main__. Foo object at 0x000002bfb7fc04e0>}#when name and sex are the same, A and B are considered the same, set to go back#comment out the __hash__ method in the classPrint(Set ([A, b]))#error Display class Foo cannot hash description of the __hash__ method of a set dependent object#comment out the __eq__ method in the classPrint(Set ([A, b]))#The result is still two elements that do not go back to the __eq__ method of the set and rely on the object to return the result

　　__len__

class Foo:     def __len__ (self):         return = Foo ()print(Len (a))  #  Ten   The built-in function Len calls the object's __len__ method, which is used by default as the __len__ method

Double underline method in Python class