Python List Slicing: Get Part of a List

Slicing: Get Part of a List

Slicing takes a part of a list. Write list[start:stop], and Python gives you a new list with the items from start up to, but not including, stop.

Think of cutting a few slices from a loaf of bread. You choose where the knife starts and where it stops, and you take the piece in between.

12days = ["Mon", "Tue", "Wed", "Thu", "Fri"]
print(days[1:4])

days[1:4] starts at index 1 ("Tue") and stops before index 4, so "Fri" is left out.

The rules

• start is included, stop is not.
• The number of items you get is stop - start. Here, 4 - 1 = 3 items.
• A slice is always a list, with brackets, even when it holds one item.

An item or a slice?

1234days = ["Mon", "Tue", "Wed", "Thu", "Fri"]
print(days[2])
print(days[2:3])
print(days[0:2])

days[2] with one number gives the item: the string Wed. days[2:3] with a colon gives a list that holds that item.

Common beginner doubt: why is stop left out?

It looks odd at first, but it makes slices fit together neatly. days[0:2] and days[2:5] share the number 2, and together they make the whole list, with no item twice and none missing. And stop - start is always the number of items.

list[start:stop] gives the items from start up to, but not including, stop.

Slice from the Start or to the End

You can leave out the start or the stop of a slice. Leave out the start, and the slice begins at the front. Leave out the stop, and it runs to the end.

Think of a race result: "the first three" get medals, and "everyone from fourth place on" gets a certificate. You don't need to say where the list begins or ends.

123scores = [90, 85, 77, 64, 50]
print(scores[:3])
print(scores[3:])

The rules

Negative numbers work in slices just like negative indexes:

123scores = [90, 85, 77, 64, 50]
print(scores[-2:])
print(scores[:-1])

Slices never give an IndexError

A slice that goes past the end stops at the end. A slice with nothing in it gives an empty list:

123names = ["Sunny", "Rocky"]
print(names[:10])
print(names[5:])

Good: names[:3] for "the top three". If there are only two names, you get both, and no error.

Avoid: names[2] when the list might be short. An index past the end is an IndexError (topic 2).

Leave out start to begin at the front. Leave out stop to go to the end.

Slice Step and Reversing a List

A slice can take a third number, the step: list[start:stop:step]. A step of 2 takes every second item. A step of -1 walks backwards, so list[::-1] gives the list in reverse order.

Think of climbing stairs two at a time. You still go from the bottom to the top, but you only step on every second stair.

1234nums = [0, 1, 2, 3, 4, 5, 6, 7]
print(nums[::2])
print(nums[1::2])
print(nums[1:6:2])

The rules

• The step is how far to jump each time. When you leave it out, it's 1.
• [::2] takes every second item, starting at the front.
• [1::2] takes every second item, starting at index 1.
• [::-1] takes every item, from the back to the front.

Reverse a list

123queue = ["Sunny", "Rocky", "Lucky"]
print(queue[::-1])
print(queue)

[::-1] makes a new, reversed list. The original queue is unchanged, as the second line shows.

Avoid: a step of 0

A step of 0 would never move, so Python refuses it:

12nums = [1, 2, 3]
print(nums[::0])

Common beginner doubt: which number is which?

With two colons, the numbers are always start, stop, step, in that order. nums[1:6:2] means "start at 1, stop before 6, jump by 2". Empty places use the defaults: the front, the end, and a step of 1.

The third number is the step: [::2] takes every second item, and [::-1] reverses the list.

A Slice Is a New List: Copy a List

Every slice is a new list. Changing the slice never changes the original. So the slice [:], with no numbers at all, is the easiest way to copy a list.

Think of a photocopy. Scribble on the copy as much as you like: the original page stays clean.

12345original = [1, 2, 3]
copy = original[:]
copy[0] = 99
print(original)
print(copy)

The surprise: = does not copy a list

1234original = [1, 2, 3]
same = original
same[0] = 99
print(original)

Wait, what? We changed same, and original changed too!

same = original doesn't make a second list. It puts a second name tag on the same list. Think of one box with two labels: whichever label you use, you open the same box.

Common beginner doubt: but chapter 4 said = copies!

In chapter 4, b = a and then a = 20 left b alone. That's still true here: a = 20 puts the label a on a new value. Here, though, same[0] = 99 doesn't move a label. It changes something inside the one list that both names share.

The rules

Slicing makes a new list, so list[:] is a copy. = only gives the same list a second name.

Practice: Slicing

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