Think Python, 3rd Edition

Chapter 10. Dictionaries

This chapter presents a built-in type called a dictionary. It is one of Python’s best features—and the building block of many efficient and elegant algorithms.

We’ll use dictionaries to compute the number of unique words in a book and the number of times each one appears. And in the exercises, we’ll use dictionaries to solve word puzzles.

A Dictionary Is a Mapping

A dictionary is like a list, but more general. In a list, the indices have to be integers; in a dictionary they can be (almost) any type. For example, suppose we make a list of number words, like this:

lst = ['zero', 'one', 'two']
       

We can use an integer as an index to get the corresponding word:

lst[1]
       

'one'
       

But suppose we want to go in the other direction, and look up a word to get the corresponding integer. We can’t do that with a list, but we can with a dictionary. We’ll start by creating an empty dictionary and assigning it to numbers:

numbers = {}
numbers
       

{}
       

The curly braces, {}, represent an empty dictionary. To add items to the dictionary, we’ll use square brackets:

numbers['zero'] = 0
       

This assignment adds to the dictionary an item, which represents the association of a key and a value. In this example, the key is the string 'zero' and the value is the integer 0. If we display the dictionary, we see that it contains one item, which contains a key and a value separated by a colon:

numbers
       

{'zero': 0}
       

We can add more items like this:

numbers['one'] = 1
numbers['two'] =