SingletonMetaClass --- Thinking in Python

#: c01:SingletonMetaClass.py
class SingletonMetaClass(type):
  def __init__(cls,name,bases,dict):
    super(SingletonMetaClass,cls).__init__(name,bases,dict)
    original_new = cls.__new__
    def my_new(cls,*args,**kwds):
      if cls.instance == None:
        cls.instance = original_new(cls,*args,**kwds)
      return cls.instance
    cls.instance = None
    cls.__new__ = staticmethod(my_new)

class bar(object):
  __metaclass__ = SingletonMetaClass
  def __init__(self,val):
    self.val = val
  def __str__(self):
    return `self` + self.val

x=bar('sausage')
y=bar('eggs')
z=bar('spam')
print x
print y
print z
print x is y is z
#<hr>
output = '''
<__main__.bar object at 0x00793100>spam
<__main__.bar object at 0x00793100>spam
<__main__.bar object at 0x00793100>spam
1
'''