Python __init__ __new__ 有和区别

[xiehongan123]

如题
另外，这两个方法的有什么作用
在Python里面有很多__开头的方法，他们一般是些什么方法？
Python的面向对象我还像有或多的困惑，怎么样才能理解清楚？能否有人传授点秘诀或是给个链接？

[notax]

一般很少情框下用到__new__，
__new__ 比 __init__ 优先

class A(object):
    def __init__(self):
        print ("__init__")

    def __new__(self):
        print ("__new__")


A()  

$ python test.py 
__new__

http://www.python.org/download/releases/2.2/descrintro/

When subclassing immutable built-in types like numbers and strings, and occasionally in other situations, the static method __new__ comes in handy. __new__ is the first step in instance construction, invoked before __init__. The __new__ method is called with the class as its first argument; its responsibility is to return a new instance of that class. Compare this to __init__: __init__ is called with an instance as its first argument, and it doesn't return anything; its responsibility is to initialize the instance. There are situations where a new instance is created without calling __init__ (for example when the instance is loaded from a pickle). There is no way to create a new instance without calling __new__ (although in some cases you can get away with calling a base class's __new__).

Recall that you create class instances by calling the class. When the class is a new-style class, the following happens when it is called. First, the class's __new__ method is called, passing the class itself as first argument, followed by any (positional as well as keyword) arguments received by the original call. This returns a new instance. Then that instance's __init__ method is called to further initialize it. (This is all controlled by the __call__ method of the metaclass, by the way.)

Here is an example of a subclass that overrides __new__ - this is how you would normally use it.

    >>> class inch(float):
    ...     "Convert from inch to meter"
    ...     def __new__(cls, arg=0.0):
    ...         return float.__new__(cls, arg*0.0254)
    ...
    >>> print inch(12)
    0.3048
    >>> 

This class isn't very useful (it's not even the right way to go about unit conversions) but it shows how to extend the constructor of an immutable type. If instead of __new__ we had tried to override __init__, it wouldn't have worked:

    >>> class inch(float):
    ...     "THIS DOESN'T WORK!!!"
    ...     def __init__(self, arg=0.0):
    ...         float.__init__(self, arg*0.0254)
    ...
    >>> print inch(12)
    12.0
    >>> 

The version overriding __init__ doesn't work because the float type's __init__ is a no-op: it returns immediately, ignoring its arguments.

All this is done so that immutable types can preserve their immutability while allowing subclassing. If the value of a float object were initialized by its __init__ method, you could change the value of an existing float object! For example, this would work:

    >>> # THIS DOESN'T WORK!!!
    >>> import math
    >>> math.pi.__init__(3.0)
    >>> print math.pi
    3.0
    >>>

[Semigod]

很好，学习了一下

[一土草三工]

mark xuexi

[xs810559195]

__开头的方法是私有方法，别的类用不了

[zfzaizheli]

不懂不懂，了解中。。。

[jamseyang]

学习下。。。

[四哥]

__new__一般是用于继承内置类的，返回值是一个对象，__init__只做一些初始化工作，没有返回值

[tomsheep]

mark

[vanta]

我一般都是用__init__，在python里面似乎很少要用到__new__，

[amu9900]

__new__：创建对象时调用，返回当前对象的一个实例相当于java里面的构造器差不多

__init__：创建完对象后调用，对当前对象的实例的一些初始化，无返回值

如果重写了__new__而在__new__里面没有调用__init__那么__init__将不起作用。

可以这样理解，默认是创建（__new__），然后调用__init__，所以一楼的例子举错了。

[notax]

楼上说的没错，先谢谢啦


如果 __new__ 有return instance 的话，应该是等於call 了__init__

http://docs.python.org/reference/datamodel.html#object.__new__

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__().

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

[ultimatebuster]

其实所有的python function都返回NONE，除非你特殊指出return

[龙根]

不懂，顶下

[xiehongan123]

谢谢大家，

[firmlyjin]

very good

[wahaha00239]

3Q,有了一份理解了

[国谦哥哥]

学习了。多谢。