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-- flyaflya [<<DateTime(2005-08-04T09:09:28Z)>>]
<<TableOfContents>>
== 模式名称 ==
=== 意图 ===
保证一个类仅有一个实例，并提供一个访问它的全局访问点。

=== 代码 ===
{{{
#!python
class Singleton:
    """ A python singleton """

    class __impl:
        """ Implementation of the singleton interface """

        def spam(self):
            """ Test method, return singleton id """
            return id(self)

    # storage for the instance reference
    __instance = None

    def __init__(self):
        """ Create singleton instance """
        # Check whether we already have an instance
        if Singleton.__instance is None:
            # Create and remember instance
            Singleton.__instance = Singleton.__impl()

        # Store instance reference as the only member in the handle
        self.__dict__['_Singleton__instance'] = Singleton.__instance

    def __getattr__(self, attr):
        """ Delegate access to implementation """
        return getattr(self.__instance, attr)

    def __setattr__(self, attr, value):
        """ Delegate access to implementation """
        return setattr(self.__instance, attr, value)


# Test it
s1 = Singleton()
print id(s1), s1.spam()

s2 = Singleton()
print id(s2), s2.spam()

# Sample output, the second (inner) id is constant:
# 8172684 8176268
# 8168588 8176268
}}}
=== 另一个实现代码 ===

从[[http://members.chello.nl/f.niessink/|TaskCoach]]的代码中摘抄，因此许可证为GPL。
-- QiangningHong



{{{
#!python
class Singleton(type):
    """Singleton Metaclass"""

    def __init__(cls, name, bases, dic):
        super(Singleton, cls).__init__(name, bases, dic)
        cls.instance = None

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

使用方法:
{{{
#!python
class MyClass(object):
    __metaclass__ = Singleton

ob1 = MyClass()
ob2 = MyClass()
assert ob1 is ob2
}}}

=== 例子 ===
## 实用的代码演示
'''声明'''
{{{#!python
}}}
'''使用情景'''

{{{
#!python

}}}

=== 特殊说明 ===
 * 例如：MFC中的App类，一般程序中用到的全局变量。