class duck():
  def walk(self):
    print('I walk like a duck')
  def swim(self):
    print('i swim like a duck')

class person():
  def walk(self):
    print('this one walk like a duck') 
  def swim(self):
    print('this man swim like a duck')

def watch_duck(animal):
  animal.walk()
  animal.swim()

small_duck = duck()
watch_duck(small_duck)

output  
I walk like a duck
i swim like a duck


duck_like_man = person()
watch_duck(duck_like_man)

output  
this one walk like a duck
this man swim like a duck


class Lame_Foot_Duck():
  def swim(self):
    print('i am lame but i can swim')

lame_duck = Lame_Foot_Duck()
watch_duck(lame_duck)

output 
AttributeError: Lame_Foot_Duck instance has no attribute 'walk'

class CollectionClass():
  lists = [1,2,3,4]
  def __getitem__(self, index):
    return self.lists[index]

iter_able_object = CollectionClass()

class Another_iterAbleClass():
  lists=[1,2,3,4]
  list_position = -1

  def __iter__(self):
    return self

  def next(self): #还有更简单的实现，使用生成器或迭代器什么的:)
    self.list_position += 1
    if self.list_position >3:
      raise StopIteration
    return self.lists[self.list_position]

another_iterable_object=Another_iterAbleClass()

print(iter_able_object[1])
print(iter_able_object[1:3])
output
2
[2, 3]

another_iterable_object[2]
output
Traceback (most recent call last):
 File "/Users/steinliber/a.py", line 32, in <module>
  another_iterable_object[2]
TypeError: 'Another_iterAbleClass' object does not support indexing

print(next(another_iterable_object))
output
1
print(next(another_iterable_object))
output
2

print(next(iter_able_object))
output
Traceback (most recent call last):
 File "/Users/steinliber/a.py", line 29, in <module>
  print(next(iter_able_object))
TypeError: IterAbleClass object is not an iterator