>>> classmates=['XiaoMing','Bob','LiLei']

>>> classmates

['XiaoMing', 'Bob', 'LiLei']

>>> len(classmates)

3

>>> classmates[0]

'XiaoMing'

>>> classmates[1]

'Bob'

>>> classmates[2]

'LiLei'

>>> classmates[3]

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

IndexError: list index out of range

>>> classmates[-1]

'LiLei'

>>> classmates[-2]

'Bob'

>>> classmates[-3]

'XiaoMing'

>>>