#: c01:SingletonPattern.py

class OnlyOne:
  class __OnlyOne:
    def __init__(self, arg):
      self.val = arg
    def __str__(self):
      return `self` + self.val
  instance = None
  def __init__(self, arg):
    if not OnlyOne.instance:
      OnlyOne.instance = OnlyOne.__OnlyOne(arg)
    else:
      OnlyOne.instance.val = arg
  def __getattr__(self, name):
    return getattr(self.instance, name)

x = OnlyOne('sausage')
print x
y = OnlyOne('eggs')
print y
z = OnlyOne('spam')
print z
print x
print y
print `x`
print `y`
print `z`
output = '''
<__main__.__OnlyOne instance at 0076B7AC>sausage
<__main__.__OnlyOne instance at 0076B7AC>eggs
<__main__.__OnlyOne instance at 0076B7AC>spam
<__main__.__OnlyOne instance at 0076B7AC>spam
<__main__.__OnlyOne instance at 0076B7AC>spam
<__main__.OnlyOne instance at 0076C54C>
<__main__.OnlyOne instance at 0076DAAC>
<__main__.OnlyOne instance at 0076AA3C>
'''
#:~