Slicing: Taking Part of a List

Why this matters

You often want a piece of a list: the first page of results, the last few messages, every other row. A slice does that in one short expression, and it works on strings too.

The idea

letters = ["a", "b", "c", "d", "e", "f"]
print(letters[1:4])    # ['b', 'c', 'd']   (index 1 up to, but not including, 4)
print(letters[:2])     # ['a', 'b']        (leave out the start: begin at 0)
print(letters[4:])     # ['e', 'f']        (leave out the stop: go to the end)
print(letters[-2:])    # ['e', 'f']        (the last two)
print(letters[::2])    # ['a', 'c', 'e']   (every second item)
print(letters[::-1])   # ['f', 'e', 'd', 'c', 'b', 'a']  (backwards)

The pattern is [start:stop:step]. The stop index is not included, so letters[1:4] has 4 - 1 = 3 items. A slice never causes an IndexError: if you ask for too much, you just get what exists.

The same works on text: "python"[:2] is "py", and "python"[::-1] is "nohtyp".

Try it

Predict the result of letters[2:5], letters[:-1] and "hello"[1:3], then run them to check.

Your turn

The starter reads whole numbers into a list called numbers. Use slices to print these four lines (the example is for 1 2 3 4 5 6 7):

First three: [1, 2, 3]
Last two: [6, 7]
Every other: [1, 3, 5, 7]
Reversed: [7, 6, 5, 4, 3, 2, 1]

The checks use lists of different lengths, so do not write the numbers out by hand.

Recap

• items[start:stop:step] gives a new list; stop is excluded.
• Leave a part out to mean “from the beginning” or “to the end”; negative numbers count from the end.
• [::-1] reverses, and slices also work on strings.

Common mistakes

• Off-by-one with stop: items[0:3] has three items (indexes 0, 1, 2), not four.
• Confusing an index with a slice: items[-1] is one item, while items[-1:] is a list holding that item.
• Expecting a slice to change the original. It makes a new list.