Python提示类型错误

list_side=[3,4,5,6,6,6,4,4,3]
list_side1=[list_side[i,i+2] for i in range(0,7,3)]
print(list_side1)

list_side=[3,4,5,6,6,6,4,4,3]
list_side1=[list_side[i] for i in range(0,7,3)]
print(list_side1)