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

# 我這里只寫了2個類，但是其實有很多類
for i in [A, B]:
  test = get_test(i)
  # 我要判斷以下是否獲得了這個類方法才能決定是否可以執(zhí)行
  if test:
    test()

但是我用Null方法就可以這樣

class Null(object):

  def __init__(self, *args, **kwargs):
    "忽略參數(shù)"
    return None

  def __call__(self, *args, **kwargs):
    "忽略實例調(diào)用"
    return self

  def __getattr__(self, mname):
    "忽略屬性獲得"
    return self

  def __setattr__(self, name, value):
    "忽略設(shè)置屬性操作"
    return self

  def __delattr__(self, name):
    '''忽略刪除屬性操作'''
    return self

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

  def __str__(self):
    return "Null"

還是上面的功能

class Null(object):

  def __init__(self, *args, **kwargs):
    "忽略參數(shù)"
    return None

  def __call__(self, *args, **kwargs):
    "忽略實例調(diào)用"
    return self

  def __getattr__(self, mname):
    "忽略屬性獲得"
    return self

  def __setattr__(self, name, value):
    "忽略設(shè)置屬性操作"
    return self

  def __delattr__(self, name):
    '''忽略刪除屬性操作'''
    return self

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

  def __str__(self):
    return "Null"

橋接模式
這個模式其實就是把產(chǎn)品類的實現(xiàn)和抽象類分離，能夠靈活的變化，假如你記得狀態(tài)模式,它是修改內(nèi)部屬性，而橋接模式是指定好內(nèi)部屬性，每個產(chǎn)品類指定這個屬性被橋接模式類調(diào)用，適用于產(chǎn)品類可能經(jīng)常調(diào)整變化，這樣還能減少了產(chǎn)品類之間的耦合

python的例子
這里實現(xiàn)一個打印操作系統(tǒng)名字的功能

class Bridge(object):

  def __init__(self):
    self.__implementation = None

  def someFunctionality(self):
    raise NotImplemented()

class UseCase1(Bridge):
  # 根據(jù)初始化參數(shù)傳入實現(xiàn)的產(chǎn)品類
  def __init__(self, implementation):
    self.__implementation = implementation
  # 根據(jù)傳入的產(chǎn)品類的屬性打印結(jié)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

# 這里其實才是實現(xiàn)的產(chǎn)品類
class Linux(ImplementationInterface):

  # 它定義了這個方法，回應操作系統(tǒng)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()

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