Dictionaries in Python

Python dictionaries are another collection. Real word dictionaries are a good analogy to understand them: they contain a list of items, each item has a key and a value.

In the traditional dictionary, the key is the word and the value is the explanation or description of it. In Python you can do something similar.

Related course: Complete Python Programming Course & Exercises

Example

Introduction

In a more strict way of speaking (mathematical), a dictionary is a one-to-one mapping. For every key in the dictionary there is a value. This value is assigned to the key.

dictionary: a set which contains (key, value) pairs

For every key in a dictionary, there is a value. Unlike lists, dictionaries do not have an specific order.

That means if you were to define a dictionary and loop over it, each time the output could be different because Python neglects the order.

Defintion

Lets type some code! You can create a dictionary with a one liner. A dictionary is defined with these brackets {}.

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words = {}

Beware: This is just two characters different from a list. Making a typo here, would mix it up with a list.

Then you can create a mapping. A mapping is defined as a key,value pair. First define the key then the value:

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words[key] = value

Dictionary example

In the example below we define some key,value pairs and then print them using their unique keys.
Type the code below, save it and run it:

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#!/usr/bin/python

words = {}
words["BMP"] = "Bitmap"
words["BTW"] = "By The Way"
words["BRB"] = "Be Right Back"

print words["BMP"]
print words["BRB"]

If you are a beginner, then I highly recommend this YouTube Python Playlist.

Exercise

1. Make a mapping from countries to country short codes
2. Print each item (key and value)