Python自动化之高级语法单例模式

方法1 共享属性;所谓单例就是所有引用(实例、对象)拥有相同的状态(属性)和行为(方法)

class Borg(object):
_state = {}
def __new__(cls, *args, **kw):
ob = super(Borg, cls).__new__(cls, *args, **kw)
ob.__dict__ = cls._state
return ob

class MyClass2(Borg):
a = 1

one = MyClass2()
two = MyClass2()

#one和two是两个不同的对象,id, ==, is对比结果可看出
two.a = 3
print one.a
#3
print id(one)
#28873680
print id(two)
#28873712
print one == two
#False
print one is two
#False
#但是one和two具有相同的（同一个__dict__属性）,见:
print id(one.__dict__)
#30104000
print id(two.__dict__)
#30104000

始终是第一个实例

class Singleton(object):
''''' A python style singleton '''
def __new__(cls, *args, **kw):
if not hasattr(cls, '_instance'):
org = super(Singleton, cls)
cls._instance = org.__new__(cls, *args, **kw)
return cls._instance

if __name__ == '__main__':
class SingleSpam(Singleton):
def __init__(self, s):
self.s = s
def __str__(self):
return self.s
s1 = SingleSpam('spam')
print id(s1), s1
s2 = SingleSpam('spa')
print id(s2), s2
print id(s1), s1