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 rating: 314

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

Answer rating: 192

Python sequence slice addresses can be written as a[start:end:step] and any of start, stop or end can be dropped. a[::3] is every third element of the sequence.