syntax - What is :: (double colon) in Python when subscripting sequences?

I know that I can use something like string[3:4] to get a substring in Python, but what does the 3 mean in somesequence[::3]?

Answer

it means 'nothing for the first argument, nothing for the second, and jump by three'. It gets every third item of the sequence sliced.
Extended slices is what you want. New in Python 2.3