newsingleton.py

this is the most basic to learn python

#: c01:NewSingleton.py

class OnlyOne(object):
  class __OnlyOne:
    def __init__(self):
      self.val = None
    def __str__(self):
      return `self` + self.val
  instance = None
  def __new__(cls): # __new__ always a classmethod
    if not OnlyOne.instance:
      OnlyOne.instance = OnlyOne.__OnlyOne()
    return OnlyOne.instance
  def __getattr__(self, name):
    return getattr(self.instance, name)
  def __setattr__(self, name):
    return setattr(self.instance, name)

x = OnlyOne()
x.val = 'sausage'
print x
y = OnlyOne()
y.val = 'eggs'
print y
z = OnlyOne()
z.val = 'spam'
print z
print x
print y
#<hr>
output = '''
<__main__.__OnlyOne instance at 0x00798900>sausage
<__main__.__OnlyOne instance at 0x00798900>eggs
<__main__.__OnlyOne instance at 0x00798900>spam
<__main__.__OnlyOne instance at 0x00798900>spam
<__main__.__OnlyOne instance at 0x00798900>spam
'''
#:~