In python, _ init _, _ new _, and _ call _ summary __init ___ new __

In python, _ init _, _ new _, and _ call _ summary __init ___ new __
This article mainly introduces _ init _, _ new _, and _ call _ in python. For more information, see

1. _ new _ (cls, * args, ** kwargs) is called when an object is created, and an instance of the current object is returned. Note: The first parameter here is cls, that is, the class itself.
2. _ init _ (self, * args, ** kwargs) is called after an object is created. It initializes some instances of the current object and has no return value, that is, after _ new _ is called, it is initialized Based on the returned instance. Note that the first parameter here is self, which is the object itself. [note the difference with new]
3. _ call _ (self, * args, ** kwargs) if the class implements this method, it is equivalent to using this type of object as a function, which is equivalent to reloading the brackets operator.
 

Take a look at the specific example:

Copy the Code as follows:
Class O (object ):

Def _ init _ (self, * args, ** kwargs ):
Print "init"
Super (O, self). _ init _ (* args, ** kwargs)

Def _ new _ (cls, * args, ** kwargs ):
Print "new", cls
Return super (O, cls). _ new _ (cls, * args, ** kwargs)

Def _ call _ (self, * args, ** kwargs ):
Print "call"

Oo = O ()
Print "________"
Oo ()

Printed:
Copy the Code as follows:
New
Init
________
Call
For example, if Python Singleton (Singleton mode) is implemented, can we just reload some _ new _ methods?
Copy the Code as follows:
Class Singleton1 (object ):
"Reload the new method """
Def _ new _ (cls, * args, ** kwargs ):
If not "_ instance" in vars (cls ):
Cls. _ instance = super (Singleton1, cls). _ new _ (cls, * args, ** kwargs)
Return cls. _ instance

Can we reload the _ init _ method? Obviously not. Because the _ init _ method was called before, an object has been generated and the singleton mode cannot be implemented.

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Note 1, _ Init _ is not equivalent to the constructor in C,Instance constructedCome out.

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class A(object):    def __init__(self,name):        self.name=name    def getName(self):        return 'A '+self.name

When we execute

1	a=A('hello')

Can be understood

12	a=object.__new__(A)A.__init__(a,'hello')

That is, _ init _ initializes the instantiated object.

Note 2Subclass canDo not override_ Init __, when instantiating a subclass,Will be called automatically_ Init _ defined in the superclass __

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class B(A):    def getName(self):        return 'B '+self.name if __name__=='__main__':    b=B('hello')    print b.getName()

However, ifRewritten_ Init __, when instantiating a subclassIt will not be called implicitly._ Init _ defined in the superclass __

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class C(A):    def __init__(self):        pass    def getName(self):        return 'C '+self.name if __name__=='__main__':    c=C()    print c.getName()

The error "AttributeError: 'C' object has no attribute 'name'" will be reported, soIf _ init __is overwritten, it is best to explicitly call the _ init _ method of the superclass to be able to use or extend the behavior in the superclass.

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class C(A):    def __init__(self,name):        super(C,self).__init__(name)    def getName(self):        return 'C '+self.name if __name__=='__main__':    c=C('hello')        print c.getName()