常见的在Python中实现单例模式的三种方法

class Singleton(object):
 
  def __new__(cls, *args, **kwargs):
    if not hasattr(cls, '_instance'):
      orig = super(Singleton, cls)
      cls._instance = orig.__new__(cls, *args, **kwargs)
    return cls._instance
 
class MyClass(Singleton):
  a = 1
 
one = MyClass()
two = MyClass()
 
#one和two完全相同,可以用id(), ==, is检测
print id(one)  # 29097904
print id(two)  # 29097904
print one == two  # True
print one is two  # True

class Singleton2(type):
 
  def __init__(cls, name, bases, dict):
    super(Singleton2, cls).__init__(name, bases, dict)
    cls._instance = None
 
  def __call__(cls, *args, **kwargs):
    if cls._instance is None:
      cls._instance = super(Singleton2, cls).__call__(*args, **kwargs)
    return cls._instance
 
class MyClass2(object):
  __metaclass__ = Singleton2
  a = 1
 
one = MyClass2()
two = MyClass2()
 
print id(one)  # 31495472
print id(two)  # 31495472
print one == two  # True
print one is two  # True

def singleton(cls, *args, **kwargs):
  instances = {}
  def _singleton():
    if cls not in instances:
      instances[cls] = cls(*args, **kwargs)
    return instances[cls]
  return _singleton
 
@singleton
class MyClass3(object):
  a = 1
 
one = MyClass3()
two = MyClass3()
 
print id(one)  # 29660784
print id(two)  # 29660784
print one == two  # True
print one is two  # True