如何在python中创建带参数的单例类

how to create singleton class with arguments in python

我正在寻找创建在第一次创建时接受参数的单例类的正确方法。我的研究将我引向三种不同的方式：

Metaclass

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class Singleton(type):
instance = None
def __call__(cls, *args, **kwargs):
if cls.instance is None:
cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
return cls.instance

class ASingleton(metaclass=Singleton):
pass

__new__

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class Singleton(object):
instance = None
def __new__(cls, *args, **kwargs):
if cls.instance is None:
cls.instance = super().__new__(cls, *args, **kwargs)
return cls.instance

Decorator

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def Singleton(myClass):
instances={}
def getInstance(*args, **kwargs):
if myClass not in instances:
instances[myClass] = myClass(*args, **kwargs)
return instances[myClass]
return getInstance

@Singleton
class SingletonTest(object):
pass

所有这些方法都可以很好地工作，但是当涉及到初始化(比如在普通类中使用uuinit_uuu)时，我无法找到正确的方法来实现它。我唯一能想到的解决方案是用这种方式使用元类方法：

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class Singleton(type):
instance = None

def __call__(cls, *args, **kwargs):
if cls.instance is None:
cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
return cls.instance

class ASingleton(metaclass=Singleton):
def __init__(self,j):
self.j=j

我不知道这是否是创建接受参数的singleton的正确方法。

相关讨论

除了@andriyivaneyko的答案，这里还有一个线程安全的元类singleton实现：

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# Based on tornado.ioloop.IOLoop.instance() approach.
# See https://github.com/facebook/tornado
# Whole idea for this metaclass is taken from: https://stackoverflow.com/a/6798042/2402281
class ThreadSafeSingleton(type):
_instances = {}
_singleton_lock = threading.Lock()

def __call__(cls, *args, **kwargs):
# double-checked locking pattern (https://en.wikipedia.org/wiki/Double-checked_locking)
if cls not in cls._instances:
with cls._singleton_lock:
if cls not in cls._instances:
cls._instances[cls] = super(ThreadSafeSingleton, cls).__call__(*args, **kwargs)
return cls._instances[cls]

class YourImplementation(metaclass=ThreadSafeSingleton):
def __init__(self, *args, **kwargs):
pass # your implementation goes here

希望你觉得它有用！

相关讨论

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class OnlyOne:
class __OnlyOne:
def __init__(self, arg):
self.val = arg
def __str__(self):
return repr(self) + self.val
instance = None
def __init__(self, arg):
if not OnlyOne.instance:
OnlyOne.instance = OnlyOne.__OnlyOne(arg)
else:
OnlyOne.instance.val = arg
def __getattr__(self, name):
return getattr(self.instance, name)

用法

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x = OnlyOne('sausage')
print(x)
y = OnlyOne('eggs')
print(y)
z = OnlyOne('spam')
print(z)
print(x)
print(y)
print(`x`)
print(`y`)
print(`z`)
output = '''
<__main__.__OnlyOne instance at 0076B7AC>sausage
<__main__.__OnlyOne instance at 0076B7AC>eggs
<__main__.__OnlyOne instance at 0076B7AC>spam
<__main__.__OnlyOne instance at 0076B7AC>spam
<__main__.__OnlyOne instance at 0076B7AC>spam
<__main__.OnlyOne instance at 0076C54C>
<__main__.OnlyOne instance at 0076DAAC>
<__main__.OnlyOne instance at 0076AA3C>
'''