装饰器单例

def singleton(cls):
...     instances = {}
...     def getinstance(*args, **kw):
...         if cls not in instances:
...             instances[cls] = cls(*args, **kw)
...         return instances[cls]
...     return getinstance
... 
... @singleton
... class MyClass:
...   pass
... 
c = MyClass()
c1 = MyClass()
type(c)
<class '__main__.MyClass'>
type(c1)
<class '__main__.MyClass'>
id(c)
4358766264
id(c1)
4358766264

singleton中instances已经超出了原函数的作用域，在getinstance函数中还能使用，这是闭包的特性。

把instances从字典类型改成None，会出现报错在 local 变量中不到instances！

def singleton1(cls):
...     instances = None
...     def getinstance(*args, **kw):
...         if instances is None:
...             instances = cls(*args, **kw)
...         return instances
...     return getinstance
... 
... @singleton1
... class MyClass1:
...   pass
... 
mc1 = MyClass1() 
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "<input>", line 4, in getinstance
UnboundLocalError: local variable 'instances' referenced before assignment

这是为什么？希望大佬说说