Python编写只允许实例化一个对象的类

>>> class T: __total = 0 def __init__(self, value): if T.__total != 0: raise Exception('You can only create one instanse') self.value = value T.__total += 1

>>> t1 = T(3)

# 实例化第二个对象时出错 >>> t2 = T(5) Traceback (most recent call last): File "<pyshell#8>", line 1, in <module> t2 = T(5) File "<pyshell#6>", line 5, in __init__ raise Exception('You can only create one instanse') Exception: You can only create one instanse

但是，由于Python对私有数据成员没有提供严格的访问控制保护机制，所以上面的代码无法避免下面的情况。

>>> T._T__total = 0 >>> t2 = T(5) >>> T._T__total = 0 >>> t3 = T(8) >>> t1 <__main__.T object at 0x0000018771177FD0> >>> t2 <__main__.T object at 0x0000018771177F98> >>> t3 <__main__.T object at 0x00000187711F8F98>