Python单例模式防止构造函数重复调用
python中一般我们会使用内置放方法__new__来实现单例
class Single(object):
_instance = None
def __new__(cls, *args, **kw):
if cls._instance is None:
cls._instance = object.__new__(cls, *args, **kw)
return cls._instance
def __init__(self):
print('init')
以上代码有个缺陷，就是每次实例化后都会调用构造函数__init__, 我们想要的效果一般是第二次以后就直接返回之前的实例了，这才是单例嘛。
In [16]: for x in range(5):
...: _ = Single()
...:
init
init
init
init
init
In [17]:
我们可以引入一个类属性来标志是否就行初始化，改造后代码如下
class Single(object):
_instance = None
_is_init = False
def __new__(cls, *args, **kwargs):
if cls._instance is None:
cls._instance = object.__new__(cls, *args, **kwargs)
return cls._instance
def __init__(self):
if Single._is_init:
return
print('init')
Single._is_init = True
验证
In [18]: for x in range(5):
...: _ = Single()
...:
init
In [19]:
我们也可以通过类装饰器来实现单例
class Singleton(object):
def __init__(self, cls):
self._cls = cls
self._instance = {}
def __call__(self):
if self._cls not in self._instance:
self._instance[self._cls] = self._cls()
return self._instance[self._cls]
@Singleton
class Cls2(object):
def __init__(self):
print('init')
验证
In [20]: for x in range(5):
...: _ = Cls2()
...:
init
In [21]: