Python完成单例模式的5种方式

写在前面

学究嘛，就记录一下;
实质全是根据设定一个标志位来完成, 通俗化的讲便是当第一次实例化时, 记录下"早已实例化了", 当再度实例化时, 将"记录"的详细地址回到;
题外话, 涵数它不香么? 还面向对象编程, 要什么目标.

1. 模块(module)

• 这类方法无需做什么, 一切正常写code就可以了, 一旦这一模块被导进便会形成.pyc文件, 第二次导进时载入的便是.pyc

  # girl.py
  class GirlFriend:
      pass
  
  Lucy = GirlFriend()
  
  # me.py
  from girl import Lucy
  

2. 装饰器完成

• 界定一个装饰器以下

  def singleton(cls):
      instances = {}
      @functools.wraps(cls)
      def _wrapper(*args, **kw):
          if cls not in instances:
              print('沒有该案例	，建立一个')
              instances[cls] = cls(*args, **kw)
          return instances[cls]
      return _wrapper
  

• 应用装饰器

  @singleton
  class GirlFriend:
      pass
  

3.

类

装饰器使其

class Singleton:
    def __init__(self, cls):
        self._cls = cls
        self._instance = {}

    def __call__(self, *args):
        if self._cls not in self._instance:
            self._instance[self._cls] = self._cls(*args)
        return self._instance[self._cls]

# 用法
@Singleton
class GirlFriend:
    pass

4.

__new__

方式完成

class GirlFriend:

    def __new__(cls, *args, **kw):
        if not hasattr(cls, '_instance'):
            cls._instance = super().__new__(cls)
        return cls._instance

# 用法
Lucy = GirlFriend()

5. 元类完成

class GirlFriend(type):
    def __new__(cls, name, bases, attrs):
        cls._instance = None
        return type.__new__(cls, name, bases, attrs)

    def __call__(cls, *args, **kw):
        if cls._instance is None:
            cls._instance = type.__call__(cls, *args, **kw)
        return cls._instance

# 用法
class Lucy(metaclass=GirlFriend):
    pass

汇总

1. 装饰器完成非常简单, 也罢了解