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

> type(MyClass)
type
> type(type)
type

def update_(name, bases, dct):
  attrs = ((name, value) for name, value in dct.items() if not name.startswith('__'))
  uppercase_attr = {name.upper(): value for name, value in attrs}
  return type(name, bases, uppercase_attr)


class Singleton(object):
  __metaclass__ = update_
  abc = 2

d = Singleton()
print d.ABC
# 2

def __init__(cls, what, bases=None, dict=None): # known special case of type.__init__
    """
    type(object) -> the object's type
    type(name, bases, dict) -> a new type
    # (copied from class doc)
    """
    pass

@staticmethod # known case of __new__
def __new__(S, *more): # real signature unknown; restored from __doc__
  """ T.__new__(S, ...) -> a new object with type S, a subtype of T """
  pass

def __call__(self, *more): # real signature unknown; restored from __doc__
  """ x.__call__(...) <==> x(...) """
  pass

class Basic(type):
  def __new__(cls, name, bases, newattrs):
    print "new: %r %r %r %r" % (cls, name, bases, newattrs)
    return super(Basic, cls).__new__(cls, name, bases, newattrs)

  def __call__(self, *args):
    print "call: %r %r" % (self, args)
    return super(Basic, self).__call__(*args)

  def __init__(cls, name, bases, newattrs):
    print "init: %r %r %r %r" % (cls, name, bases, newattrs)
    super(Basic, cls).__init__(name, bases, dict)


class Foo:
  __metaclass__ = Basic

  def __init__(self, *args, **kw):
    print "init: %r %r %r" % (self, args, kw)

a = Foo('a')
b = Foo('b')

new: <class '__main__.Basic'> 'Foo' () {'__module__': '__main__', '__metaclass__': <class '__main__.Basic'>, '__init__': <function init at 0x106fd5320>}
init: <class '__main__.Foo'> 'Foo' () {'__module__': '__main__', '__metaclass__': <class '__main__.Basic'>, '__init__': <function init at 0x106fd5320>}
call: <class '__main__.Foo'> ('a',)
init: <__main__.Foo object at 0x106fee990> ('a',) {}
call: <class '__main__.Foo'> ('b',)
init: <__main__.Foo object at 0x106feea50> ('b',) {}