Pyhton Singleton模式-优快云博客

本文介绍了一种使用装饰器而非元类实现Python单例模式的方法。该方法限制较少且易于实现，但装饰后的类不能被继承。通过示例展示了如何正确获取单例实例。

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class Singleton:
	"""
	A non-thread-safe helper class to ease implementing singletons.
	This should be used as a decorator -- not a metaclass -- to the
	class that should be a singleton.

	The decorated class can define one `__init__` function that
	takes only the `self` argument. Other than that, there are
	no restrictions that apply to the decorated class.

	To get the singleton instance, use the `Instance` method. Trying
	to use `__call__` will result in a `TypeError` being raised.

	Limitations: The decorated class cannot be inherited from.

	"""

	def __init__(self, decorated):
		self._decorated = decorated

	def Instance(self):
		"""
		Returns the singleton instance. Upon its first call, it creates a
		new instance of the decorated class and calls its `__init__` method.
		On all subsequent calls, the already created instance is returned.

		"""
		try:
			return self._instance
		except AttributeError:
			self._instance = self._decorated()
			return self._instance

	def __call__(self):
		raise TypeError('Singletons must be accessed through `Instance()`.')

	def __instancecheck__(self, inst):
		return isinstance(inst, self._decorated)

示例

 @Singleton
   class Foo:
       def __init__(self):
           print 'Foo created'

   f = Foo() # Error, this isn't how you get the instance of a singleton

   f = Foo.Instance() # Good. Being explicit is in line with the Python Zen
   g = Foo.Instance() # Returns already created instance

   print f is g # True

原文地址： http://stackoverflow.com/questions/42558/python-and-the-singleton-pattern