Partially unpacking a sequence

[tkpmep at hotmail.com]

I have a list y
>>>y
['20001201', 'ARRO', '04276410', '18.500', '19.500', '18.500',
'19.500', '224']

from which I want to extract only the 2nd and 4th item by partially
unpacking the list. So I tried
>>>a,b = y[2,4]
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
TypeError: list indices must be integers

Out of curiosity, I tried
>>>a,b = y[2:4]
>>>a
'04276410'
>>> b
'18.500'

Why does this work (to a point  - it gives me items 2 and 3, not 2 and
4 as I require) and not my first attempt? What is the right syntax to
use when partially upacking a sequence?

Thanks in advance

Thomas Philips