用空对象来取代客户端对Null对象的检查

优点:

  1. 能够实现对Null对象的自定义控制
  2. 加强系统的稳定性

Demo

  1. """
  2. 空对象模式
  3. """
  4. from abc import ABCMeta, abstractmethod
  7. class AbstractCustomer(metaclass=ABCMeta):
  8.     @abstractmethod
  9.     def is_nil(self):
  10.         pass
  12.     def get_name(self):
  13.         pass
  16. class RealCustomer(AbstractCustomer):
  17.     def __init__(self, name):
  18.         self.name = name
  20.     def is_nil(self):
  21.         return False
  23.     def get_name(self):
  24.         return self.name
  27. class NullCustomer(AbstractCustomer):
  28.     def is_nil(self):
  29.         return True
  31.     def get_name(self):
  32.         return 'Not Available in Customer Database'
  35. class CustomerFactory:
  36.     names = {"Rob", "Joe", "Julie"}
  38.     @classmethod
  39.     def get_customer(cls, name):
  40.         for value in cls.names:
  41.             if str.upper(value) == str.upper(name):
  42.                 return RealCustomer(name)
  44.         return NullCustomer()
  47. if __name__ == '__main__':
  48.     c_1 = CustomerFactory.get_customer('rob')
  49.     c_2 = CustomerFactory.get_customer('ccc')
  51.     print(c_1.get_name())
  52.     print(c_2.get_name())