This is a Slice.

[start:end:step]

Leaving any blank puts them at a default value, in your case it is taking every y elements starting at x and going until the end of the list.

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

This is a Slice.

[start:end:step]

Leaving any blank puts them at a default value, in your case it is taking every y elements starting at x and going until the end of the list.

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