举例讲解Python中的Null模式与桥接模式编程

class A(object):
  pass

class B(object):
  b = 1
  @classmethod
  def test(cls):
    print cls.b

def get_test(x):
  try:
    return x.test
  except AttributeError: 
    return None

# 我这里只写了2个类，但是其实有很多类
for i in [A, B]:
  test = get_test(i)
  # 我要判断以下是否获得了这个类方法才能决定是否可以执行
  if test:
    test()

class Null(object):

  def __init__(self, *args, **kwargs):
    "忽略参数"
    return None

  def __call__(self, *args, **kwargs):
    "忽略实例调用"
    return self

  def __getattr__(self, mname):
    "忽略属性获得"
    return self

  def __setattr__(self, name, value):
    "忽略设置属性操作"
    return self

  def __delattr__(self, name):
    '''忽略删除属性操作'''
    return self

  def __repr__(self):
    return "<Null>"

  def __str__(self):
    return "Null"

class Null(object):

  def __init__(self, *args, **kwargs):
    "忽略参数"
    return None

  def __call__(self, *args, **kwargs):
    "忽略实例调用"
    return self

  def __getattr__(self, mname):
    "忽略属性获得"
    return self

  def __setattr__(self, name, value):
    "忽略设置属性操作"
    return self

  def __delattr__(self, name):
    '''忽略删除属性操作'''
    return self

  def __repr__(self):
    return "<Null>"

  def __str__(self):
    return "Null"

class Bridge(object):

  def __init__(self):
    self.__implementation = None

  def someFunctionality(self):
    raise NotImplemented()

class UseCase1(Bridge):
  # 根据初始化参数传入实现的产品类
  def __init__(self, implementation):
    self.__implementation = implementation
  # 根据传入的产品类的属性打印结果
  def someFunctionality(self):
    print "UseCase1: ",
    self.__implementation.anotherFunctionality()


class UseCase2(Bridge):
  def __init__(self, implementation):
    self.__implementation = implementation

  def someFunctionality(self):
    print "UseCase2: ",
    self.__implementation.anotherFunctionality()


class ImplementationInterface:

  def anotherFunctionality(self):
    raise NotImplemented

# 这里其实才是实现的产品类
class Linux(ImplementationInterface):

  # 它定义了这个方法，回应操作系统的名字
  def anotherFunctionality(self):
    print "Linux!"


class Windows(ImplementationInterface):
  def anotherFunctionality(self):
    print "Windows."


def main():
  linux = Linux()
  windows = Windows()

  useCase = UseCase1(linux)
  useCase.someFunctionality()

  useCase = UseCase1(windows)
  useCase.someFunctionality()

  useCase = UseCase2(linux)
  useCase.someFunctionality()

  useCase = UseCase2(windows)
  useCase.someFunctionality()


if __name__ == "__main__":
  main()