Slicing

Taking a piece

A slice gives you a new list containing part of the original:

numbers = [0, 1, 2, 3, 4, 5]
numbers[1:4]    # [1, 2, 3]

Read [start:stop] as "from start, up to but not including stop" — the same rule as range(). A slice of [1:4] gives you three items, because 4 − 1 is 3.

Either end can be left out:

numbers[:3]     # [0, 1, 2] — from the beginning
numbers[3:]     # [3, 4, 5] — to the end
numbers[:]      # a copy of the whole list

A third number is the step, and [::-1] is the idiomatic way to reverse:

numbers[::2]    # [0, 2, 4]
numbers[::-1]   # [5, 4, 3, 2, 1, 0]

Slicing works on strings too, and never raises IndexError — an out-of-range slice just gives you less.

Your turn: take the first three, the last two, and a reversed copy.

Slicing in a function

Slices are the tidiest way to answer "the middle bit" questions without index arithmetic scattered through your code.

Your turn: write middle(items) returning everything except the first and last items. For a list of two or fewer, return an empty list.