Python 设计模式
系列 - Python 技巧
目录
单例模式
实现方式
- 模块实现单例
# a.py
class Singleton:
pass
singleton = Singleton()
# b.py
from a import singleton- 装饰器实现单例
def singleton(cls):
_instance = {}
def decorated(*args, **kwargs):
if cls not in _instance:
_instance[cls] = cls(*args, **kwargs)
return _instance[cls]
return decorated- 使用类的方式创建装饰器
class Singleton(object):
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]- 使用类方法实现单例
class Person(object):
@classmethod
def get_instance(cls, *args, **kwargs):
if not hasattr(cls, '_instance'):
cls._instance = cls(*args, **kwargs)
return cls._instance- 使用
__new__方法实现单例
class Person(object):
def __new__(cls, *args, **kwargs):
if not hasattr(cls, '_instance'):
cls._instance = super().__new__(cls)
return cls._instance- 使用元类
metaclass实现单例
class Singleton(type):
def __call__(cls, *args, **kwargs):
if not hasattr(cls, "_instance"):
cls._instance = super().__call__(*args, **kwargs)
return cls._instance
class Person(metaclass=Singleton):
def __init__(self,name, age):
self.name = name
self.age = age
Warning
在多线程和异步的环境下，有可能出现多个线程同时拿到_instance=None这个值，导致创建多个实例，解决办法就是通过锁来限制。
- 多线程
class Person(object):
_lock = threading.Lock()
@classmethod
def get_instance(cls, *args, **kwargs):
with cls._lock:
if not hasattr(cls, '_instance'):
cls._instance = cls(*args, **kwargs)
return cls._instance- 异步
class Person(object):
_lock = asyncio.Lock()
@classmethod
def get_instance(cls, *args, **kwargs):
async with cls._lock:
if not hasattr(cls, '_instance'):
cls._instance = cls(*args, **kwargs)
return cls._instance