"""访问者模式"""
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支持多重继承，在解析父类的__init__时，定义解析顺序的是子类的__mro__属性，内容为一个存储要解析类顺序的元组。"""
  """观察到，super的执行路径和类的__mro__列举的类顺序吻合；而__mro__的顺序可以看作是深搜的结果"""
  for cls in node.__class__.__mro__:
   """方法名"""
   meth_name = 'visit_' + cls.__name__


   """getattr()函数是Python自省的核心函数，具体使用大体如下：
    获取对象引用getattr,Getattr用于返回一个对象属性，或者方法

    如果Visitor对象中有属性meth_name则获得方法返回的值，否则赋值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('通常访问: ' + node.__class__.__name__)

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

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