# 使用new关键字实现

class A(object):

__instance = None

def __new__(cls, *args, **kwargs):

if cls.__instance is None:

cls.__instance = object.__new__(cls)

return cls.__instance

else:

return cls.__instance

def __init__(self):

pass

A1 = A()

A2 = A()

print(id(A1) == id(A2))

# 使用装饰器

def singleton(cls):

__instance = {}

def inner():

if cls not in __instance:

__instance[cls] = cls()

return __instance[cls]

return inner

@singleton

class B(object):

def __init__(self):

pass

B1 = B()

B2 = B()

print(id(B1) == id(B2))