Python_中__init__和__new__的异同

1：__new__:它是创建对象时调用，会返回当前对象的一个实例；

     __init__:它是创建对象后调用，对当前对象的一些实例初始化，无返回值

代码示例：

>>> class Data(object):
    def __init__(cls):
        cls.x = 2
        print "init"
        return cls    ###在init中不可用

    
>>> data = Data()
init

Traceback (most recent call last):
  File "<pyshell#100>", line 1, in <module>
    data = Data()
TypeError: __init__() should return None, not 'Data'

>>> class Data(object):
    def __new__(cls):
        cls.x = 3
        print "new"
        return cls

    
>>> data = Data()
new
>>> data.x
3

由上可见，__new__方法会返回所构造的对象，__init__则不会，__init__无返回值

2：再类中，若__new__和__init__同时存在时，先调用__new__

代码示例：

>>> class Data(object):
    def __new__(self):
        print "new"
    def __init__(self):
        print "init"

        
>>> data = Data()
new

3：If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to __new__().

如果__new__返回一个对象的实例，会隐式调用__init__

If __new__() does not return an instance of cls, then the new instance’s __init__() method will not be invoked.

如果__new__不返回一个对象的实例，__init__不会被调用

代码示例：

>>> class A(object):
    def __new__(Class):
        object = super(A,Class).__new__(Class)
        print "in new"
        return object   ##返回对象的实例
    def __init__(self):
        print "in init"

        
>>> A()
in new
in init
<__main__.A object at 0x0215CD70>
>>> class B(object):
    def __new__(cls):
        print "in new"
        return cls
    def __init__(self):
        print "in init"

        
>>> b=B()
in new