cls.__instance = cls.__new__(cls, *args, **kwargs)
return cls.__instance
if __name__ == '__main__':
foo = Foo()
错误如下：
RuntimeError: maximum recursion depth exceeded in cmp
而这样也有一样的错误：
class Foo(object):
__instance = None
def __init__(self):
if self.__class__.__instance == None:
self.__class__.__instance = Foo()
print 'init'
if __name__ == '__main__':
foo = Foo()
该怎么做呢？
下面参考了http://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python/31887#31887：
class Foo(object):
__instance = None
def __new__(cls, *args, **kwargs):
print 'hhhhhhhhh'
if not cls.__instance:
cls.__instance = super(Foo, cls).__new__(cls, *args, **kwargs)
return cls.__instance
def hi(self):
print 'hi, world'
print 'hi, letian'
if __name__ == '__main__':
foo1 = Foo()
foo2 = Foo()
print id(foo1)
print id(foo2)
print isinstance(foo1, object)
print isinstance(foo1, Foo)
foo1.hi()
运行结果：
hhhhhhhhh
hhhhhhhhh
39578896
39578896
True
True
hi, world
hi, letian
那么，到底发生了什么，我们先回顾一下super：
>>> print super.__doc__
super(type) -> unbound super object
super(type, obj) -> bound super object; requires isinstance(obj, type)
super(type, type2) -> bound super object; requires issubclass(type2, type)
Typical use to call a cooperative superclass method:
class C(B):
def meth(self, arg):
super(C, self).meth(arg)
可以肯定上面的单例模式代码中的这一行代码：
cls.__instance = super(Foo, cls).__new__(cls, *args, **kwargs)
super(Foo, cls)是object，super(Foo, cls).new方法使用的是object的new方法。我们看一下object.new方法的作用：
>>> print object.__new__.__doc__
T.__new__(S, ...) -> a new object with type S, a subtype of T
如果是一个继承链
class Fo(object):
