Lists

So far every variable has held exactly one value. Often, though, there are many values of the same kind: the temperatures of a week, all the shopping, the names in a group. Creating a variable of its own for each would be tedious, and the program would not know that they belong together. That is what lists are for: they keep many values under one name, in a fixed order, like the carriages of a train from the basic ideas.

Creating a list

A list stands in square brackets, and the values in it are called elements and are separated by commas:

temperatures = [12.5, 14.0, 9.5, 16.0, 13.5]
shopping = ["milk", "bread", "apples"]
empty = []

print(temperatures)
print(shopping)
print(len(shopping))

You already know len() from text; with a list it counts the elements. A list may also be empty, and it may hold elements of different types as well. Mostly, though, a list holds values of the same kind, because you want to do the same with them.

Reaching an element

Every element has a position, its index, and Python starts counting at 0. The first element therefore has the index 0, the second the index 1. With a negative index you count from the back: -1 is the last element.

shopping = ["milk", "bread", "apples"]

print(shopping[0])
print(shopping[1])
print(shopping[-1])

If you ask for a position that does not exist, Python reports an error. Change a line in the box above to print(shopping[3]) and look at the message: the list has three elements at the positions 0, 1 and 2, and IndexError: list index out of range says that 3 lies beyond them.

You can also replace an element by assigning a new value to its position:

shopping = ["milk", "bread", "apples"]
shopping[1] = "rolls"
print(shopping)

Calculating with all values at once

For lists of numbers, Python comes with functions that sum up all the elements:

temperatures = [12.5, 14.0, 9.5, 16.0, 13.5]

print(sum(temperatures))
print(min(temperatures))
print(max(temperatures))

average = sum(temperatures) / len(temperatures)
print(f"On average {average:.1f} degrees")

There is no function of its own for the average, but it is quickly worked out: the sum divided by the number of values.

Changing a list

A list can grow and shrink. append() adds an element at the end, and remove() takes out the first element with the given value:

shopping = ["milk", "bread", "apples"]

shopping.append("cheese")
print(shopping)

shopping.remove("bread")
print(shopping)

append and remove are methods like upper() for text and are attached to the name with a dot in the same way. Unlike the methods of text, however, the methods of a list change the list itself: you do not write shopping = shopping.append("cheese") but only shopping.append("cheese").

Whether a value occurs in the list you ask with in. The answer is True or False, a truth value with which you make decisions in part 2 of this path (Comparisons and truth values):

shopping = ["milk", "bread", "apples"]

print("bread" in shopping)
print("butter" in shopping)

By the way, remove() with a value that is not in the list stops with a ValueError.

Sorting

sorted() returns a sorted version of the list, numbers by size and text in alphabetical order. The original list stays as it was:

temperatures = [12.5, 14.0, 9.5, 16.0, 13.5]

print(sorted(temperatures))
print(sorted(temperatures, reverse=True))
print(temperatures)

With reverse=True, sorted() sorts in descending order, with the largest value first.

From text to list and back

Several values often arrive as a single text, for instance from an input. split() cuts a text at a separator into a list of texts, and join() puts a list of texts back together into one text:

line = "milk, bread, apples, cheese"
shopping = line.split(", ")
print(shopping)
print(len(shopping))

print(" and ".join(shopping))

Note that join() is attached to the separator and the list stands in the brackets, not the other way round. Without anything in the brackets, split() cuts at spaces.

What is still missing

With a list you can now hold many values, pick out single ones and sum up all of them. What you still lack is a way of doing something of its own with each element, such as converting every temperature to Fahrenheit or writing every item of shopping on a line of its own. That is the job of loops, and they follow in part 3 of this path with Loops with for.

Takeaways

• A list stands in square brackets and holds many values in a fixed order.
• The index starts at 0, [-1] is the last element, and a position outside the list gives an IndexError.
• len(), sum(), min() and max() sum up a list; the average is sum(...) / len(...).
• append() and remove() change the list itself, in checks whether a value is in it, sorted() returns a sorted version.
• split() turns a text into a list, join() turns a list into a text.