class Singleton(object):
  _instance = None

  def __new__(cls, *args):
    if not isinstance(cls._instance, cls):
      cls._instance = super(Singleton, cls).__new__(cls, *args)
    return cls._instance

class D(Singleton):

  def __new__(cls, *args):
    return super(D, cls).__new__(cls, *args)

def singleton(_cls):
  inst = {}

  def getinstance(*args, **kwargs):
    if _cls not in inst:
      inst[_cls] = _cls(*args, **kwargs)
    return inst[_cls]
  return getinstance

@singleton
class MyClass(object):
  pass

class Singleton(type):
  _inst = {}
  
  def __call__(cls, *args, **kwargs):
    if cls not in cls._inst:
      cls._inst[cls] = super(Singleton, cls).__call__(*args)
    return cls._inst[cls]


class MyClass(object):
  __metaclass__ = Singleton

class IOLoop(object):

  @staticmethod
  def instance():
    """Returns a global `IOLoop` instance.

Most applications have a single, global `IOLoop` running on the
main thread. Use this method to get this instance from
another thread. To get the current thread's `IOLoop`, use `current()`.
"""
    if not hasattr(IOLoop, "_instance"):
      with IOLoop._instance_lock:
        if not hasattr(IOLoop, "_instance"):
          # New instance after double check
          IOLoop._instance = IOLoop()
    return IOLoop._instance

class Singleton(object):

  @staticmathod
  def instance():
    if not hasattr(Singleton, '_instance'):
      Singleton._instance = Singleton()
    return Singleton._instance

class Singleton(object):

  def __new__(cls, *args, **kwargs):
    if not hasattr(cls, '_instance'):
      cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
    return cls._instance

import threading

class Singleton(object):

  _instance_lock = threading.Lock()
  
  @staticmethod
  def instance():
    with Singleton._instance_lock:
      if not hasattr(Singleton, '_instance'):
        Singleton._instance = Singleton()
    return Singleton._instance

class Singleton {

  private static Singleton instance = new Singleton();
  
  private Singleton() {}
  
  public static Singleton getInstance() {
    return instance;
  }
  
}

class Singleton(object):

  @staticmethod
  def instance():
    return _g_singleton

_g_singleton = Singleton()

# def get_instance():
# return _g_singleton

import functools

def singleton(cls):
  ''' Use class as singleton. '''

  cls.__new_original__ = cls.__new__

  @functools.wraps(cls.__new__)
  def singleton_new(cls, *args, **kw):
    it = cls.__dict__.get('__it__')
    if it is not None:
      return it

    cls.__it__ = it = cls.__new_original__(cls, *args, **kw)
    it.__init_original__(*args, **kw)
    return it

  cls.__new__ = singleton_new
  cls.__init_original__ = cls.__init__
  cls.__init__ = object.__init__

  return cls

#
# Sample use:
#

@singleton
class Foo:
  def __new__(cls):
    cls.x = 10
    return object.__new__(cls)

  def __init__(self):
    assert self.x == 10
    self.x = 15

assert Foo().x == 15
Foo().x = 20
assert Foo().x == 20

def singleton(cls):
  instance = cls()
  instance.__call__ = lambda: instance
  return instance

#
# Sample use
#

@singleton
class Highlander:
  x = 100
  # Of course you can have any attributes or methods you like.

Highlander() is Highlander() is Highlander #=> True
id(Highlander()) == id(Highlander) #=> True
Highlander().x == Highlander.x == 100 #=> True
Highlander.x = 50
Highlander().x == Highlander.x == 50 #=> True