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): #還有更簡單的實現，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