The underlying implementation of Python classes and objects

According to the "Principle of Everything" in Python, all created objects are the result of a known class instantiation, and which class is instantiated by? First look at the following piece of code

Class Foo (object):    passobj = Foo () print type (obj) print type (Foo) result is: <class ' __main__. Foo ' ><type ' type ' >

As you can see: obj is instantiated by foo, and Foo is created by the type class

So foo can do this, look at the following:

def func:    print "charles" Foo = Type (' Foo ', (object,), {' Func ': func}) Print foof = foo () F.func () The result is: <class ' _ _main__. Foo ' >charles

So the question arises, since class is instantiated by type, how does the class implement the creation of classes?

In fact, in a class instantiated by type, there is a __metaclass__ field that indicates who is instantiated to get the class

Class MyType (Type):    def __init__ (self, What, Bases=none, dict=none):        print "333"        super (MyType, self). __ Init__ (What, bases, Dict)    def __call__ (self, *args, **kwargs):        print "444"        obj = self.__new__ (self, *args, * *kwargs)        self.__init__ (obj) class Foo (object):    __metaclass__ = MyType    def __init__ (self):        print ' 222 '    def __new__ (CLS, *args, **kwargs):        print ' 1111 '        return object.__new__ (CLS, *args, **kwargs) obj = Foo () The result is: 3334441111222

You can see that both MyType and Foo have __new__ () methods, so what does this method do?

__new__ () function:

See below:

Class A (object):    def __init__ (self):        print ' init '    def __new__ (CLS, *args, **kwargs):        print "first "        print object.__new__ (Cls,*args,**kwargs) A () Result: first<__main__. A Object at 0x0000000002730eb8>

You can see that the __new__ method executes first than the __init__ method, object.__new__ (Cls,*args,**kwargs) is an instantiated object of a, if I print self

Class A (object):    def __init__ (self):        print self, '----'        print ' init '    def __new__ (CLS, *args, **kwargs) :        print "First"        print object.__new__ (Cls,*args,**kwargs)        return object.__new__ (Cls,*args,**kwargs) A () The result is: first<__main__. A object at 0x000000000260be10><__main__. A object at 0x000000000260be10>----init

You can see the value of self, which is exactly the result of __new__ return;

Observe the object returned by __new__, which contains the CLS, which represents the current class, and if it is not the current class (even the parent class), __init__ will not be called:

Class B (object):    passclass A (b):    def __init__ (self):        print self, '----'        print ' init '    def __new_ _ (CLS, *args, **kwargs):        print "First"        return object.__new__ (B,*args,**kwargs) A () result is: first<__main__. A Object at 0x000000000238beb8>

　　

Conclusion: 1, examples of Python classes are created by the __new__ method.

2. The __new__ method must have a CLS parameter that represents the current class, which is called only if the current cls,__init__ is passed in.

3, the class instance self, is the __new__ method return object.__new__ (Cls,*args,**kwargs) the result;

Reference: http://www.cnblogs.com/tuzkee/p/3540293.html

Http://www.cnblogs.com/wupeiqi/p/4766801.html

　　

The underlying implementation of Python classes and objects