Lists

Introduction

A list holds many values in order, inside square brackets. Unlike a string, a list can be changed after it is created, which makes it powerful and, sometimes, surprising.

Programs rarely handle one value at a time. They deal with all the marks in a class or every item in a cart. A list lets one name stand for many values, and a single loop can handle all of them.

How it works

Making and using a list

marks = [72, 85, 64]
marks.append(90)
print(marks[0], marks[-1])
print(len(marks), sum(marks))

In the objects panel the list is drawn as a row of boxes with their positions (0, 1, 2, 3), and the variable marks has an arrow pointing to it. The variable does not hold the list itself; it points to it.

• marks[0] is the first item and marks[-1] the last.
• append(90) adds an item at the end. Watch the box gain a fourth cell.
• len() counts the items and sum() adds them up.

Two names, one list

a = [1, 2, 3]
b = a
c = a.copy()
b.append(4)
print(a, c)

This is the most important picture in this lesson. After b = a, two arrows point to the same list. So b.append(4) changes the list that a points to as well. a.copy() makes a separate list, so c keeps [1, 2, 3].

Syntax

marks = [72, 85, 64]
marks[0]              # first item
marks[-1]             # last item
marks.append(90)
len(marks), sum(marks), max(marks)
copy = marks.copy()

Example

nums = [4, 9, 2, 7]
biggest = nums[0]
for x in nums:
    if x > biggest:
        biggest = x
print(biggest)

This finds the largest number by hand. biggest starts as the first item; then every item is compared with it, and the calculation box shows each check: 4 > 4, 9 > 4, 2 > 9, 7 > 9. Python's built-in max(nums) does the same job in one call.

Line by line

1. nums is the list to examine.
2. biggest = nums[0] takes the first item as the best so far.
3. for x in nums: visits each item.
4. If x is larger than the best so far, biggest becomes x.
5. After the loop, biggest holds the largest item, and it is printed.

Expected result

9

In the real world

A feed of posts, the rows of a table and the results of a search are all lists. The sharing rule causes real bugs: a function that changes a list it was given also changes the caller's list, which is why careful code makes a copy first.

Common mistakes

• marks[4] on a list of four items is an IndexError; the last position is 3.
• Expecting b = a to copy a list. It only adds a second name; use a.copy() or list(a).
• Adding or removing items from a list while looping over it. Loop over a copy instead.

Best practices

• Keep items of the same kind in one list.
• Use for x in items unless you need the position.
• Copy a list before changing it if someone else may still use it.

Summary

A list holds values in order and can change. Positions start at 0. Variables point to lists, so assigning a list makes a second name for it, and copy() makes a separate list.

Practice

Read five marks on one line, store them in a list with split() and int(), and print the average with two decimal places.

Mark lesson complete