singleton.py

#!/usr/bin/python
# -*- coding : utf-8 -*-

"""
brief: 
	Singleton.
"""

flag = 0
if flag == 0:
  # Using __dict__.
  # Different objects have the same state. 
  class Singleton(object):
    __shared_state = {}
    def __init__(self):
      self.__dict__ = self.__shared_state
      self.state = 'Init'
    def __str__(self):
      return self.state
	
elif flag == 1:
  # Using __new__.
  # The same object with the same state.
  class Singleton(object):
    def __init__(self):
      self.state = 'Init'
    def __str__(self):
      return self.state	
    def __new__(cls):
      if not hasattr(cls, 'instance'):
        cls.instance = super(Singleton, cls).__new__(cls)
      return cls.instance
	
elif flag == 2:
  # Using decorator.
  # The same object with the same state.
  def decorator(cls):
    instances = {}
    def getinstance(*args,**kwargs):
      if cls not in instances:
        instances[cls] = cls(*args,**kwargs)
      return instances[cls]
    return getinstance
    
  # decorator(Singleton(object))
  @decorator
  class Singleton(object):
    def __init__(self):
      self.state = 'Init'
    def __str__(self):
      return self.state
		
if __name__ == '__main__':
  obj1 = Singleton()
  obj2 = Singleton()
	
  obj1.state = 'a'
  obj2.state = 'b'

  print('obj1: {0}'.format(obj1))
  print('obj2: {0}'.format(obj2))

  obj2.state = 'c'

  print('obj1: {0}'.format(obj1))
  print('obj2: {0}'.format(obj2))

  print('- obj1 id: {0}'.format(id(obj1)))
  print('- obj2 id: {0}'.format(id(obj2)))