方法一：使用装饰器
装饰器维护一个字典对象instances，缓存了所有单例类，只要单例不存在则创建，已经存在直接返回该实例对象。
def singleton(cls):
instances = {}
#看到有人在这有疑问，为啥instances会有缓存功能？函数调用结束时所有变量不是释放？
#正解：只有程序运行结束时退出才会释放所有资源，函数运行时会创建一个函数作用域，instances就是在当前函数
#singleton作用域内，此处调用了两次，因此在第二次判断的时候instances里面缓存了上一次函数运行时产生的数据
def wrapper(*args, **kwargs):
if cls not in instances:
instances[cls] = cls(*args, **kwargs)
return instances[cls]
return wrapper
@singleton
class Foo(object):
pass
foo1 = Foo()
foo2 = Foo()
print foo1 is foo2
方法二：使用基类
__new__是真正创建实例对象的方法，所以重写基类的__new__方法，以此来保证创建对象的时候只生成一个实例
<span class="k">class</span> <span class="n">Singleton</span>(<span class="n">object</span>):
<span class="n">def</span> <span class="n">__new__</span>(<span class="n">cls</span>, *<span class="n">args</span>, **<span class="n">kwargs</span>):
<span class="k">if</span> <span class="nb">not</span> <span class="n">hasattr</span>(<span class="n">cls</span>, <span class="s">'_instance'</span>):
<span class="n">cls</span>.<span class="n">_instance</span> = <span class="n">super</span>(<span class="n">Singleton</span>, <span class="n">cls</span>).<span class="n">__new__</span>(<span class="n">cls</span>, *<span class="n">args</span>, **<span class="n">kwargs</span>)
<span class="k">return</span> <span class="n">cls</span>.<span class="n">_instance</span>
<span class="k">class</span> <span class="n">Foo</span>(<span class="n">Singleton</span>):
<span class="nb">pass</span>
<span class="n">foo1</span> = <span class="n">Foo</span>()
<span class="n">foo2</span> = <span class="n">Foo</span>()
<span class="nb">print</span> <span class="n">foo1</span> <span class="k">is</span> <span class="n">foo2</span> <span class="c c-Singleline"># True</span>
方法三：使用元类
元类（参考：深刻理解Python中的元类）是用于创建类对象的类，类对象创建实例对象时一定会调用__call__方法，因此在调用__call__时候保证始终只创建一个实例即可，type是python中的一个元类。
<span class="k">class</span> <span class="n">Singleton</span>(<span class="n">type</span>):
<span class="n">def</span> <span class="n">__call__</span>(<span class="n">cls</span>, *<span class="n">args</span>, **<span class="n">kwargs</span>):
<span class="k">if</span> <span class="nb">not</span> <span class="n">hasattr</span>(<span class="n">cls</span>, <span class="s">'_instance'</span>):
<span class="n">cls</span>.<span class="n">_instance</span> = <span class="n">super</span>(<span class="n">Singleton</span>, <span class="n">cls</span>).<span class="n">__call__</span>(*<span class="n">args</span>, **<span class="n">kwargs</span>)
<span class="k">return</span> <span class="n">cls</span>.<span class="n">_instance</span>
<span class="k">class</span> <span class="n">Foo</span>(<span class="n">object</span>):
<span class="n">__metaclass__</span> = <span class="n">Singleton</span>
<span class="n">foo1</span> = <span class="n">Foo</span>()
<span class="n">foo2</span> = <span class="n">Foo</span>()
<span class="nb">print</span> <span class="n">foo1</span> <span class="k">is</span> <span class="n">foo2</span> <span class="c c-Singleline"># True</span>
转载自 https://foofish.net/python_singleton.html并且适当修改注释
