Python单体模式的几种常见实现方法详解

class Borg:
  _shared_state = {}
  def __init__(self):
    self.__dict__ = self._shared_state
class Singleton(Borg):
  def __init__(self, name):
    super().__init__()
    self.name = name
  def __str__(self):
    return self.name
x = Singleton('sausage')
print(x)
y = Singleton('eggs')
print(y)
z = Singleton('spam')
print(z)
print(x)
print(y)

class Singleton(type):
  _instance = None
  def __call__(self, *args, **kw):
    if self._instance is None:
      self._instance = super().__call__(*args, **kw)
    return self._instance
class MyClass(object):
  __metaclass__ = Singleton
print(MyClass())
print(MyClass())

class Singleton(type):
  _instance = None
  def __new__(cls, name, bases, dct):
    if cls._instance is None:
      cls._instance = super().__new__(cls, name, bases, dct)
    return cls._instance
class MyClass(object):
  __metaclass__ = Singleton
print(MyClass())
print(MyClass())

class Singleton(object):
  _instance = None
  def __new__(cls):
    if cls._instance is None:
      cls._instance = super().__new__(cls)
    return cls._instance
class MyClass(object):
  __metaclass__ = Singleton
print(MyClass())
print(MyClass())

class Singleton:
  _instance = None
  @classmethod
  def create(cls):
    if cls._instance is None:
      cls._instance = cls()
    return cls._instance
  def __init__(self):
    self.x = 5    # or whatever you want to do
sing = Singleton.create()
print(sing.x) # 5
sec = Singleton.create()
print(sec.x) # 5