Dictionary Basics

Dictionaries in Python allow us to work with data that has a key: value relationship, with the key in each dictionary being unique and immutable (cannot be modified).

Creating a Dictionary

An empty dictionary can be created by a pair of empty curly braces.

empty_dict = {}
print(empty_dict)

{}

The dict() function produces the same result.

empty_dict = dict()
print(empty_dict)

{}

Expectedly, an empty dictionary is not very interesting. Let’s create a dictionary to store the ice cream preferences of Mary, Jane and June.

The items in a dictionary are enclosed in curly braces, with each key: value pair seperated by a colon.

preferences = {'Mary': 'vanilla', 
               'Jane': 'chocolate',
               'June': 'chocolate'}
print(preferences)

{'Mary': 'vanilla', 'Jane': 'chocolate', 'June': 'chocolate'}

Accessing Data in a Dictionary

To retrive the girls’ names, we can use the keys() function.

print(preferences.keys())

dict_keys(['Mary', 'Jane', 'June'])

values() allows us to retrieve the flavors.

print(preferences.values())

dict_values(['vanilla', 'chocolate', 'chocolate'])

If we want to iterate on each key-value pair, we can use the items() function.

for key, value in preferences.items():
    print('Key: ' + key)
    print('Value: ' + value + '\n')

Key: Mary
Value: vanilla

Key: Jane
Value: chocolate

Key: June
Value: chocolate

You can access values directly if you know the key.

print(preferences['Mary'])

vanilla

Removing Data from a Dictionary

There are two ways that we can remove data from a dictionary. The first is by using the del keyword.

preferences['John'] = 'strawberry'
print(preferences['John'])

strawberry

del preferences['John']
print(preferences)

{'Mary': 'vanilla', 'Jane': 'chocolate', 'June': 'chocolate'}

The second way is by using the pop() function. The key difference is that pop() returns the value of the key that was removed.

preferences['John'] = 'strawberry'
print(preferences['John'])

strawberry

print('Result of pop function: ' + preferences.pop('John'))
print(preferences)

Result of pop function: strawberry
{'Mary': 'vanilla', 'Jane': 'chocolate', 'June': 'chocolate'}

Handling Missing Keys

When we attempt to access a non-existent key in the dictionary, it will result in a KeyError. This is certainly not a desired behavior in any production system.

Let’s assume that we are given a list of words and tasked to count the number of times each word appears in the list. How can we do this using a dictionary, and without running into a KeyError?

Below, you will see two ways that we can achieve this.

words = ['apple', 'apple', 'apple', 'banana', 'banana', 'cucumber']
word_counts = {}

for word in words:
    word_counts.setdefault(word, 0)
    word_counts[word] += 1

print(word_counts)

{'apple': 3, 'banana': 2, 'cucumber': 1}

An alternative method will be to create a defaultdict (from Python’s in-built collection of high performance data structures)

from collections import defaultdict

words = ['apple', 'apple', 'apple', 'banana', 'banana', 'cucumber']
word_counts = defaultdict(int)

for word in words:
    word_counts[word] += 1

print(word_counts)

defaultdict(<class 'int'>, {'apple': 3, 'banana': 2, 'cucumber': 1})