import threading
class Signleton(object):
  def __init__(self):
    print("__init__ method called")
  def __new__(cls):
    print("__new__ method called")
    mutex=threading.Lock()
    mutex.acquire() # 上鎖，防止多線程下出問(wèn)題
    if not hasattr(cls, 'instance'):
      cls.instance = super(LogSignleton, cls).__new__(cls)
    mutex.release()
    return cls.instance 
if __name__ == '__main__':
obj = Signleton()

>>> ================================ RESTART ================================
>>>
__new__ method called
__init__ method called
>>> 

print '----------------------方法1--------------------------' 
#方法1,實(shí)現(xiàn)__new__方法 
#并在將一個(gè)類的實(shí)例綁定到類變量_instance上, 
#如果cls._instance為None說(shuō)明該類還沒有實(shí)例化過(guò),實(shí)例化該類,并返回 
#如果cls._instance不為None,直接返回cls._instance 
class Singleton(object): 
  def __new__(cls, *args, **kw): 
    if not hasattr(cls, '_instance'): 
      orig = super(Singleton, cls) 
      cls._instance = orig.__new__(cls, *args, **kw) 
    return cls._instance 
 
class MyClass(Singleton): 
  a = 1 
 
one = MyClass() 
two = MyClass() 
 
two.a = 3 
print one.a 
#3 
#one和two完全相同,可以用id(), ==, is檢測(cè) 
print id(one) 
#29097904 
print id(two) 
#29097904 
print one == two 
#True 
print one is two 
#True

print '----------------------方法2--------------------------' 
#方法2,共享屬性;所謂單例就是所有引用(實(shí)例、對(duì)象)擁有相同的狀態(tài)(屬性)和行為(方法) 
#同一個(gè)類的所有實(shí)例天然擁有相同的行為(方法), 
#只需要保證同一個(gè)類的所有實(shí)例具有相同的狀態(tài)(屬性)即可 
#所有實(shí)例共享屬性的最簡(jiǎn)單最直接的方法就是__dict__屬性指向(引用)同一個(gè)字典(dict) 
#可參看:http://code.activestate.com/recipes/66531/ 
class Borg(object): 
  _state = {} 
  def __new__(cls, *args, **kw): 
    ob = super(Borg, cls).__new__(cls, *args, **kw) 
    ob.__dict__ = cls._state 
    return ob 
 
class MyClass2(Borg): 
  a = 1 
 
one = MyClass2() 
two = MyClass2() 
 
#one和two是兩個(gè)不同的對(duì)象,id, ==, is對(duì)比結(jié)果可看出 
two.a = 3 
print one.a 
#3 
print id(one) 
#28873680 
print id(two) 
#28873712 
print one == two 
#False 
print one is two 
#False 
#但是one和two具有相同的（同一個(gè)__dict__屬性）,見: 
print id(one.__dict__) 
#30104000 
print id(two.__dict__) 
#30104000 

print '----------------------方法3--------------------------' 
#方法3:本質(zhì)上是方法1的升級(jí)（或者說(shuō)高級(jí)）版 
#使用__metaclass__（元類）的高級(jí)python用法 
class Singleton2(type): 
  def __init__(cls, name, bases, dict): 
    super(Singleton2, cls).__init__(name, bases, dict) 
    cls._instance = None 
  def __call__(cls, *args, **kw): 
    if cls._instance is None: 
      cls._instance = super(Singleton2, cls).__call__(*args, **kw) 
    return cls._instance 
 
class MyClass3(object): 
  __metaclass__ = Singleton2 
 
one = MyClass3() 
two = MyClass3() 
 
two.a = 3 
print one.a 
#3 
print id(one) 
#31495472 
print id(two) 
#31495472 
print one == two 
#True 
print one is two 
#True 

print '----------------------方法4--------------------------' 
#方法4:也是方法1的升級(jí)（高級(jí)）版本, 
#使用裝飾器(decorator), 
#這是一種更pythonic,更elegant的方法, 
#單例類本身根本不知道自己是單例的,因?yàn)樗旧?自己的代碼)并不是單例的 
def singleton(cls, *args, **kw): 
  instances = {} 
  def _singleton(): 
    if cls not in instances: 
      instances[cls] = cls(*args, **kw) 
    return instances[cls] 
  return _singleton 
 
@singleton 
class MyClass4(object): 
  a = 1 
  def __init__(self, x=0): 
    self.x = x 
 
one = MyClass4() 
two = MyClass4() 
 
two.a = 3 
print one.a 
#3 
print id(one) 
#29660784 
print id(two) 
#29660784 
print one == two 
#True 
print one is two 
#True 
one.x = 1 
print one.x 
#1 
print two.x 

# mysingleton.py
class My_Singleton(object):
  def foo(self):
    pass
 
my_singleton = My_Singleton()

from mysingleton import my_singleton
my_singleton.foo()

class Singleton(object):
  _instance = None
  def __new__(cls, *args, **kw):
    if not cls._instance:
      cls._instance = super(Singleton, cls).__new__(cls, *args, **kw) 
    return cls._instance 
 
class MyClass(Singleton): 
  a = 1

>>> one = MyClass()
>>> two = MyClass()
>>> one == two
True
>>> one is two
True
>>> id(one), id(two)
(4303862608, 4303862608)

from functools import wraps
 
def singleton(cls):
  instances = {}
  @wraps(cls)
  def getinstance(*args, **kw):
    if cls not in instances:
      instances[cls] = cls(*args, **kw)
    return instances[cls]
  return getinstance
 
@singleton
class MyClass(object):
  a = 1

class Singleton(type):
  _instances = {}
  def __call__(cls, *args, **kwargs):
    if cls not in cls._instances:
      cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
    return cls._instances[cls]
 
# Python2
class MyClass(object):
  __metaclass__ = Singleton
 
# Python3
# class MyClass(metaclass=Singleton):
#  pass