Think Python, 3rd Edition

Chapter 9. Lists

This chapter presents one of Python’s most useful built-in types, lists. You will also learn more about objects and what can happen when multiple variables refer to the same object.

In the exercises at the end of the chapter, we’ll make a word list and use it to search for special words like palindromes and anagrams.

A List Is a Sequence

Like a string, a list is a sequence of values. In a string, the values are characters; in a list, they can be any type. The values in a list are called elements.

There are several ways to create a new list; the simplest is to enclose the elements in square brackets ([ and ]). For example, here is a list of two integers:

numbers = [42, 123]
       

And here’s a list of three strings:

cheeses = ['Cheddar', 'Edam', 'Gouda']
       

The elements of a list don’t have to be the same type. The following list contains a string, a float, an integer, and even another list:

t = ['spam', 2.0, 5, [10, 20]]
       

A list within another list is nested.

A list that contains no elements is called an empty list; you can create one with empty brackets, []:

empty = []
       

The len function returns the length of a list:

len(cheeses)
       

3
       

The length of an empty list is 0.

The following figure shows the state diagram for cheeses, numbers, and empty:

Lists are represented by boxes with the word “list” outside and the numbered elements of the list inside.

Lists Are Mutable ...