The example they have is not illustrative or explanatory at all.

  class Duck:

      def fly(self):  

          print("Duck flying")  



  class Sparrow:

      def fly(self):  

          print("Sparrow flying")  


  class Whale:

      def swim(self):  

          print("Whale swimming")  


  for animal in Duck(), Sparrow(), Whale():

      animal.fly()

  Duck flying  

  Sparrow flying  

  AttributeError: 'Whale' object has no attribute 'fly'