Data Structures Dictionaries

Learning objective: By the end of this lesson, students will be able to understand the concept of dictionaries in Python, differentiate them from other data structures, and demonstrate how to create and manipulate dictionaries.

What are dictionaries?

Dictionaries in Python are similar to objects in JavaScript. Dictionaries are collections of items, like objects, which are collections of properties in JavaScript.

Dictionaries have a class (type) of dict.

Basic syntax

As with objects in JavaScript, a dictionary literal is created using curly braces:

1. A key
2. A value
3. An item - or a key: value pair
4. The dictionary itself

Any immutable type (like a number) can be used as a key. Unlike in JavaScript, when strings are used as keys, they must be surrounded in quotes, as they are above. The value held by a variable can also be used as a key - this is a common pitfall for JavaScript developers new to Python:

favorite_animal = 'dog'

student = {
    'name': 'Maria',
    'favorite_integer': 5,
    favorite_animal: 'llama' # notice the lack of quotes around favorite_animal
}

print(student)
# prints: {'name': 'Maria', 'favorite_integer': 5, 'dog': 'llama'}
# note the 'dog' key - the value of the favorite_animal variable is used

Accessing items in dictionaries

Getting values

We use square brackets to get an item’s value:

name = student['name']
print(name)
# prints: Maria

Unlike JavaScript, we can’t access items in a Python dictionary using dot notation (dots are exclusively used to invoke methods on a dictionary in Python).

The get method

When attempting to access a key that does not exist in a dictionary, a KeyError will be raised.

One option to avoid this error is to use the get method:

favorite_food = student['favorite_food']
# error: KeyError: 'favorite_food'

print(student.get('favorite_food'))
# prints: None

The in operator

Another way to avoid the KeyError is to use the in operator to check if the dictionary includes a key:

if 'course' in student:
    print(f"{student['name']} is enrolled in {student['course']}")
else:
    print(f"{student['name']} is not enrolled in a course")
    # prints: Maria is not enrolled in a course

Mutation

Dictionaries are mutable:

• The values assigned to a key can be changed.
• Additional items can be added.
• Existing items can be deleted.

Setting items

We not only use square brackets to get an item’s value - we also use it to set an item’s value:

student['name'] = 'Mariana'
print(student['name'])
# prints: Mariana

Adding items

Assigning to a key that does not exist will create a new item in the dictionary.

Let’s add an item to the student dictionary:

student['age'] = 25

Deleting items

The del statement is used to delete an item from a dictionary:

del student['dog']
# verify that the item was deleted
print('dog' in student)
# prints: False

Number of items

Use the built-in len function to retrieve the number of items in a dictionary:

print(student)
# prints: {'name': 'Maria', 'favorite_integer': 5, 'age': 25}
print(len(student))
# prints: 3
print(len({}))
# prints: 0

Iterating through dictionaries

for loops are used to iterate over the items in a dictionary.

However, accessing the value of an item as follows is considered to be a Python anti-pattern:

for key in student:
    print(f"{key} is {student[key]}")
    # prints:
    # name is Maria
    # favorite_integer is 5
    # age is 25

Modifying a dictionary using this approach can lead to potential issues, which is why this is not preferred.

The preferred approach is to use the items() method to obtain a dictionary view object.

Use it in a for in loop to iterate over the view object:

for key, val in student.items():
    print(f"{key} is {val}")
    # prints:
    # name is Maria
    # favorite_integer is 5
    # age is 25

The for statement assigns values to multiple variables like with key, val above through a mechanism called tuples, discussed later.

🎓 You Do

1. Define a Python dictionary named where_my_things_are containing a few items where:
   • the keys are things you have
   • the values are the locations where you keep those things

2. Write a for loop that iterates over the items in the dictionary and prints each one as:

   My [thing] is kept [location]