【python】__new__和__init__区别

原文：http://blog.csdn.net/cnmilan/article/details/8849680

__new__：创建对象时调用，返回当前对象的一个实例
__init__：创建完对象后调用，对当前对象的实例的一些初始化，无返回值

测试1：
>>>
class A(object):
def __init__(self):
print("in init")
def
__new__(self):
print("in new")

>>> A()
in
new

测试2：
class A(object):
def __new__(Class):

Object = super(A, Class).__new__(Class)
print "in New"

return Object
def __init__(self):
print "in init"

class
B(A):
def __init__(self):
print "in B's
init"

B()
>>>
in New
in B's
init

对于New来说：
Called to create a new instance of class cls. __new__()
is a static method (special-cased so you need not declare it as such) that takes
the class of which an instance was requested as its first argument. The
remaining arguments are those passed to the object constructor expression (the
call to the class). The return value of __new__() should be the new object
instance (usually an instance ofcls).

Typical implementations create a
new instance of the class by invoking the superclass’s __new__() method using
super(currentclass, cls).__new__(cls[, ...]) with appropriate arguments and then
modifying the newly-created instance as necessary before returning
it.

这也是上面的例子2

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.

__new__()
is intended mainly to allow subclasses of immutable types (like int, str, or
tuple) to customize instance creation. It is also commonly overridden in custom
metaclasses in order to customize class creation.

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.