我只是试图简化我的一个类，并引入了一些与flyweight设计模式相同风格的功能。
然而，我有点困惑，为什么__init__总是在__new__之后被调用。我没想到会这样。有人能告诉我为什么会发生这种情况，我如何才能实现这个功能吗?(除了将实现放在__new__中，这感觉相当粗糙。)
这里有一个例子:
class A(object):
_dict = dict()
def __new__(cls):
if 'key' in A._dict:
print "EXISTS"
return A._dict['key']
else:
print "NEW"
return super(A, cls).__new__(cls)
def __init__(self):
print "INIT"
A._dict['key'] = self
print ""
a1 = A()
a2 = A()
a3 = A()
输出:
NEW
INIT
EXISTS
INIT
EXISTS
INIT
Why?
One should look at __init__ as a simple constructor in traditional OO languages. For example, if you are familiar with Java or C++, the constructor is passed a pointer to its own instance implicitly. In the case of Java, it is the this variable. If one were to inspect the byte code generated for Java, one would notice two calls. The first call is to an "new" method, and then next call is to the init method (which is the actual call to the user defined constructor). This two step process enables creation of the actual instance before calling the constructor method of the class which is just another method of that instance.
现在，在Python中，__new__是一个用户可以访问的附加功能。由于Java的类型化特性，它没有提供这种灵活性。如果一种语言提供了这种功能，那么__new__的实现者可以在返回实例之前在该方法中做很多事情，包括在某些情况下为不相关的对象创建一个全新的实例。而且，这种方法也适用于Python中的不可变类型。
当__new__返回同一类的实例时，__init__随后在返回的对象上运行。也就是说，你不能使用__new__来阻止__init__被运行。即使你从__new__返回之前创建的对象，它也会被__init__一次又一次地初始化。
下面是单例模式的通用方法，它扩展了上面的vartec答案并修复了它:
def SingletonClass(cls):
class Single(cls):
__doc__ = cls.__doc__
_initialized = False
_instance = None
def __new__(cls, *args, **kwargs):
if not cls._instance:
cls._instance = super(Single, cls).__new__(cls, *args, **kwargs)
return cls._instance
def __init__(self, *args, **kwargs):
if self._initialized:
return
super(Single, self).__init__(*args, **kwargs)
self.__class__._initialized = True # Its crucial to set this variable on the class!
return Single
完整的故事在这里。
另一种方法，实际上涉及__new__，是使用类方法:
class Singleton(object):
__initialized = False
def __new__(cls, *args, **kwargs):
if not cls.__initialized:
cls.__init__(*args, **kwargs)
cls.__initialized = True
return cls
class MyClass(Singleton):
@classmethod
def __init__(cls, x, y):
print "init is here"
@classmethod
def do(cls):
print "doing stuff"
请注意，使用这种方法，你需要用@classmethod装饰你的所有方法，因为你永远不会使用MyClass的任何真实实例。
