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Dictionaries

Understanding dictionaries allows you to model real-world objects more accurately. A dictionary in Python is a collection of key-value pairs. Each key is connected to a value, and you can use a key to access the value associated with that key.

A Simple Dictionary​

# Basic dictionary
alien = {'color': 'green', 'points': 5}

This dictionary stores two pieces of information about the alien: its color and point value.

Working with Dictionaries​

Accessing Values in a Dictionary​

alien = {'color': 'green', 'points': 5}

print(alien['color'])
print(alien['points'])

new_points = alien['points']
print(f"New points: {new_points}")

Output:

green
5
New points: 5

Adding New Key-Value Pairs​

alien = {'color': 'green', 'points': 5}

alien['x_position'] = 0
alien['y_position'] = 25
print(alien)

Output:

{'color': 'green', 'points': 5, 'x_position': 0, 'y_position': 25}

Starting with an Empty Dictionary​

alien = {}
print(alien)

Output:

{}

Modifying Values in a Dictionary​

alien = {'color': 'green', 'points': 5}

alien['color'] = 'yellow'
print(alien)

Output:

{'color': 'yellow', 'points': 5}

Removing Key-Value Pairs​

alien = {'color': 'green', 'points': 5}
print(alien)

del alien['points']
print(alien)

Output:

{'color': 'green', 'points': 5}
{'color': 'green'}

A Dictionary of Similar Objects​

favorite_languages = {
    'jen': 'python',
    'sarah': 'c',
    'edward': 'ruby',
    'phil': 'python',
}

language = favorite_languages['sarah'].title()
print(f"Sarah's favorite language is {language}.")

Output:

Sarah's favorite language is C.

Using get() to Access Values​

If you're not sure whether a key exists in a dictionary, you can use the get() method to avoid a KeyError:

# This will cause a KeyError
alien_0 = {'color': 'green', 'speed': 'slow'}
print(alien_0['points'])  # KeyError: 'points'
# Using get() to avoid errors
alien_0 = {'color': 'green', 'speed': 'slow'}

point_value = alien_0.get('points', 'No point value assigned.')
print(point_value)

Output:

No point value assigned.

The get() method syntax: dictionary.get(key, default_value)

Looping Through a Dictionary​

You can loop through all of a dictionary's key-value pairs, through its keys, or through its values.

Looping Through All Key-Value Pairs​

user_0 = {
    'username': 'efermi',
    'first': 'enrico',
    'last': 'fermi',
}

for key, value in user_0.items():
    print(f"\nKey: {key}")
    print(f"Value: {value}")

Output:

Key: username
Value: efermi

Key: first
Value: enrico

Key: last
Value: fermi

Looping Through All the Keys in a Dictionary​

favorite_languages = {
    'jen': 'python',
    'sarah': 'c',
    'edward': 'ruby',
    'phil': 'python',
}

for name in favorite_languages.keys():
    print(name.title())

Note: Looping through the keys is actually the default behavior when looping through a dictionary, so this code would have the same output:

for name in favorite_languages:
    print(name.title())

Using Keys to Check Specific People​

friends = ['phil', 'sarah']
for name in favorite_languages.keys():
    print(f"Hi {name.title()}.")
    
    if name in friends:
        language = favorite_languages[name].title()
        print(f"\t{name.title()}, I see you love {language}!")

Output:

Hi Jen.
Hi Sarah.
	Sarah, I see you love C!
Hi Edward.
Hi Phil.
	Phil, I see you love Python!

Checking Whether a Key Exists​

if 'erin' not in favorite_languages.keys():
    print("Erin, please take our poll!")

Output:

Erin, please take our poll!

Looping Through a Dictionary's Values​

favorite_languages = {
    'jen': 'python',
    'sarah': 'c',
    'edward': 'ruby',
    'phil': 'python',
}

print("The following languages have been mentioned:")
for language in favorite_languages.values():
    print(language.title())

Output:

The following languages have been mentioned:
Python
C
Ruby
Python

Avoiding Repetition with set()​

If you want to see each unique value, you can use a set:

print("The following languages have been mentioned:")
for language in set(favorite_languages.values()):
    print(language.title())

Output:

The following languages have been mentioned:
Ruby
C
Python

You can also build a set directly:

languages = {'python', 'ruby', 'c', 'python'}
print(languages)  # {'c', 'python', 'ruby'}

Nesting​

Sometimes you'll want to store multiple dictionaries in a list, or a list of items as a value in a dictionary. This is called nesting.

A List of Dictionaries​

alien_0 = {'color': 'green', 'points': 5}
alien_1 = {'color': 'yellow', 'points': 10}
alien_2 = {'color': 'red', 'points': 15}

aliens = [alien_0, alien_1, alien_2]

for alien in aliens:
    print(alien)

Output:

{'color': 'green', 'points': 5}
{'color': 'yellow', 'points': 10}
{'color': 'red', 'points': 15}

Creating a Fleet of Aliens​

# Make an empty list for storing aliens
aliens = []

# Make 30 green aliens
for alien_number in range(30):
    new_alien = {'color': 'green', 'points': 5, 'speed': 'slow'}
    aliens.append(new_alien)

# Show the first 5 aliens
for alien in aliens[:5]:
    print(alien)
print("...")

# Show how many aliens have been created
print(f"Total number of aliens: {len(aliens)}")

A List in a Dictionary​

# Store information about a pizza being ordered
pizza = {
    'crust': 'thick',
    'toppings': ['mushrooms', 'extra cheese'],
}

# Summarize the order
print(f"You ordered a {pizza['crust']}-crust pizza "
      "with the following toppings:")

for topping in pizza['toppings']:
    print(f"\t{topping}")

Output:

You ordered a thick-crust pizza with the following toppings:
	mushrooms
	extra cheese

A Dictionary in a Dictionary​

users = {
    'aeinstein': {
        'first': 'albert',
        'last': 'einstein',
        'location': 'princeton',
    },
    'mcurie': {
        'first': 'marie',
        'last': 'curie',
        'location': 'paris',
    },
}

for username, user_info in users.items():
    print(f"\nUsername: {username}")
    full_name = f"{user_info['first']} {user_info['last']}"
    location = user_info['location']
    
    print(f"\tFull name: {full_name.title()}")
    print(f"\tLocation: {location.title()}")

Output:

Username: aeinstein
	Full name: Albert Einstein
	Location: Princeton

Username: mcurie
	Full name: Marie Curie
	Location: Paris

Summary​

In this chapter you learned how to:

  • Define a dictionary and work with the information stored in it
  • Access and modify elements in a dictionary, including adding new key-value pairs and removing them
  • Use get() to access values safely
  • Loop through all key-value pairs, all keys, or all values
  • Work with lists inside dictionaries, dictionaries inside lists, and dictionaries inside dictionaries

Dictionaries let you connect pieces of related information, enabling you to model real-world objects and situations accurately. As your programs become more complex, you'll find yourself using lists and dictionaries together to structure complex data in your programs.