[Tutor] String formatting question with 's'.format()

[eizetov at gmail.com]

I'm working through the 'Learn Python' book by Mark Lutz, in this example:

>>> somelist = list('SPAM')
>>> parts = somelist[0], somelist[-1], somelist[1:3]
>>> 'first={0}, last={1}, middle={2}'.format(*parts)
"first=S, last=M, middle=['P', 'A']"

why do we need the '*' at 'parts'. I know we need it, because otherwise it  
gives an error:

Traceback (most recent call last):
File "<pyshell#17>", line 1, in <module>
'first={0}, last={1}, middle={2}'.format(parts)
IndexError: tuple index out of range

Still, wouldn't python basically see 'parts' and insert the actual tuple  
instead of the variable 'parts'? How does the machine think?
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