"""訪問者模式"""
class Node(object):
  pass

class A(Node):
  pass

class B(Node):
  pass

class C(A, B):
  pass

class Visitor(object):
  def visit(self, node, *args, **kwargs):
    meth = None
    """python支持多重繼承，在解析父類的__init__時，定義解析順序的是子類的__mro__屬性，內(nèi)容為一個存儲要解析類順序的元組。"""
    """觀察到，super的執(zhí)行路徑和類的__mro__列舉的類順序吻合；而__mro__的順序可以看作是深搜的結果"""
    for cls in node.__class__.__mro__:
      """方法名"""
      meth_name = 'visit_' + cls.__name__


      """getattr()函數(shù)是Python自省的核心函數(shù)，具體使用大體如下：
       獲取對象引用getattr,Getattr用于返回一個對象屬性，或者方法

       如果Visitor對象中有屬性meth_name則獲得方法返回的值，否則賦值None
      """
      meth = getattr(self, meth_name, None)
      if meth:
        break

    if not meth:
      meth = self.generic_visit
    return meth(node, *args, **kwargs)

  def generic_visit(self, node, *args, **kwargs):
    print('通常訪問: ' + node.__class__.__name__)

  def visit_B(self, node, *args, **kwargs):
    print('訪問_B ' + node.__class__.__name__)

a = A()
b = B()
c = C()
visitor = Visitor()
visitor.visit(a)
visitor.visit(b)
visitor.visit(c)