Python使用metaclass实现Singleton模式的方法

class Singleton(type):
  def __call__(cls, *args, **kwargs):
    print "Singleton call"
    if not hasattr(cls, 'instance'):
      cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
    return cls.instance
  def __new__(cls, name, bases, dct):
    print "Singleton new"
    return type.__new__(cls, name, bases, dct)
  def __init__(cls, name, bases, dct):
    print "Singleton init"
    super(Singleton, cls).__init__(name, bases, dct)
class Cache(object):
  __metaclass__ = Singleton
  def __new__(cls, *args, **kwargs):
    print "Cache new"
    return object.__new__(cls, *args, **kwargs)
  def __init__(cls, *args, **kwargs):
    print "Cache init"
  def __call__(cls, *args, **kwargs):
    print "Cache call"
print Cache()
print Cache()

输出：

Singleton new
Singleton init
Singleton call
Cache new
Cache init
<__main__.Cache object at 0x01CDB130>
Singleton call
<__main__.Cache object at 0x01CDB130>