python __new__和__init__区别

Python中__new__和__init__区别

__new__:创建对象时调用，会返回当前对象的一个实例

__init__:创建完对象后调用，对当前对象的一些实例初始化，无返回值

1、在类中，如果__new__和__init__同时存在，会优先调用__new__

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

2、__new__方法会返回所构造的对象，__init__则不会。__init__无返回值。

>>> class Data(object):
...     def __init__(cls):
...             cls.x = 2
...             print "init"
...             return cls
... 
>>> data = Data()
init
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() should return None, not 'Data'

>>> class Data(object):
...     def __new__(cls):
...             print "new"
...             cls.x = 1
...             return cls
...     def __init__(self):
...             print "init"
... 
>>> data = Data()
new
>>> data.x =1 
>>> data.x
1

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

<class '__main__.B'>
>>> class A(object):
...     def __new__(Class):
...             object = super(A,Class).__new__(Class)
...             print "in New"
...             return object
...     def __init__(self):
...             print "in init"
... 
>>> A()
in New
in init
<__main__.A object at 0x7fa8bc622d90>
>>> class A(object):
...     def __new__(cls):
...             print "in New"
...             return cls
...     def __init__(self):
...             print "in init"
... 
>>> a = A()      
in New

object.__init__(self[, …])
Called when the instance is created. The arguments are those passed to the class constructor expression. If a base class has an __init__() method, the derived class’s __init__() method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: BaseClass.__init__(self, [args…]). As a special constraint on constructors, no value may be returned; doing so will cause a TypeError to be raised at runtime.

在对象的实例创建完成后调用。参数被传给类的构造函数。如果基类有__init__方法，子类必须显示调用基类的__init__。

没有返回值，否则会再引发TypeError错误。

http://www.cnblogs.com/itaceo-o/p/3300289.html

本文转载自 python中 __new__和__init__区别

分类: Python