Loop through list with both content and index

Use the enumerate built-in function: http://docs.python.org/library/functions.html#enumerate

Use enumerate():

>>> S = [1,30,20,30,2]
>>> for index, elem in enumerate(S):
        print(index, elem)

(0, 1)
(1, 30)
(2, 20)
(3, 30)
(4, 2)

Like everyone else:

for i, val in enumerate(data):
    print i, val

but also

for i, val in enumerate(data, 1):
    print i, val

In other words, you can specify as starting value for the index/count generated by enumerate() which comes in handy if you don't want your index to start with the default value of zero.

I was printing out lines in a file the other day and specified the starting value as 1 for enumerate(), which made more sense than 0 when displaying information about a specific line to the user.

enumerate is what you want:

for i, s in enumerate(S):
    print s, i