1.__new__特殊方法实现
class Singleton:
def __new__(cls, *args, **kwargs):
if not hasattr(cls, '_instance'):
cls._instance = super(Singleton, cls).__new__(cls)
return cls._instance
def __init__(self, name):
self.name = name
s1 = Singleton('first')
s2= Singleton('last')
print(s1 == s2)
>> True
print(s1.name)
>> last
tips: new__方法无法避免触发__init()，初始的成员变量会进行覆盖
2.装饰器实现
def singleton(cls):
_instance = {}
def inner(*args, **kwargs):
if cls not in _instance:
_instance[cls] = cls(*args, **kwargs)
return _instance[cls]
return inner
@singleton
class Test:
def __init__(self, name):
self.name = name
t1 = Test('first')
t2 = Test('last')
print(t1==t2)
>> True
print(t2.name)
>> first
3.类装饰器实现
class Singleton:
def __init__(self, cls):
self._cls = cls
self._instance = {}
def __call__(self, *args):
if self._cls not in self._instance:
self._instance[self._cls] = self._cls(*args)
return self._instance[self._cls]
@Singleton
class Cls2:
def __init__(self, name):
self.name = name
cls1 = Cls2('first')
cls2 = Cls2('last')
print(id(cls1) == id(cls2))
>> True
print(cls1.name)
>> first
4.元类实现
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class Singleton1(type):
def __init__(self, *args, **kwargs):
self.__instance = None
super(Singleton1, self).__init__(*args, **kwargs)
def __call__(self, *args, **kwargs):
if self.__instance is None:
self.__instance = super(Singleton1, self).__call__(*args, **kwargs)
return self.__instance
class C(metaclass=Singleton1):
def __init__(self, name):
self.name = name
c1 = C('first')
c2 = C('last')
print(c1 == c2)
>> True
print(c2.name)
>> first
5.模块实现
Python 的模块就是天然的单例模式，因为模块在第一次导入时，会生成 .pyc 文件，当第二次导入时，就会直接加载 .pyc 文件，而不会再次执行模块代码。
#foo1.py
class Singleton(object):
def foo(self):
pass
singleton = Singleton()
#foo.py
from foo1 import singleton
