python __new__，,new实验实验1实验

new
实验
实验1
实验2
应用：实现单例

new

This method is only used for new-style classes (classes inheriting from object).
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 of cls).
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.
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.

实验

实验1

class A(object):    a = 1    def __new__(cls, *args, **kwargs):        print ‘+ A new‘        inst = super(A, cls).__new__(cls, *args, **kwargs)        print ‘- A new‘        #return inst    def __init__(self):        print ‘+ A init‘        print ‘- A init‘a = A()print type(a)+ A new- A new<type ‘NoneType‘>

class A(object):    a = 1    def __new__(cls, *args, **kwargs):        print ‘+ A new‘        inst = super(A, cls).__new__(cls, *args, **kwargs)        print ‘- A new‘        return inst    def __init__(self):        print ‘+ A init‘        print ‘- A init‘a = A()print type(a)结果+ A new- A new+ A init- A init<class ‘__main__.A‘>

class A(object):    _inst = None    def __new__(cls, *args, **kwargs):        print ‘+ A new‘        if not cls._inst:            print ‘do new.‘            cls._inst = super(A, cls).__new__(cls, *args, **kwargs)            cls._inst.inst_count = 0        print ‘- A new‘        return cls._inst    def __init__(self):        print ‘+ A init‘        self.inst_count += 1        print ‘- A init‘a = A()print type(a), a.inst_countaa = A()print type(aa), aa.inst_countaaa = A()print type(aaa), aaa.inst_count

+ A newdo new.- A new+ A init- A init<class ‘__main__.A‘> 1+ A new- A new+ A init- A init<class ‘__main__.A‘> 2+ A new- A new+ A init- A init<class ‘__main__.A‘> 3