Chapter 8: Dictionaries

See It in Action

Example 1: Creating and Accessing a Dictionary

Let’s create a dictionary that stores information about a student. Each piece of information has a key (like "name") and a value (like "Alice"). We use curly braces {} to define a dictionary and colons : to separate each key from its value.

# Create a dictionary for a student
student = {"name": "Alice", "age": 15, "year": 10}

# Print the whole dictionary
print(student)

# Access individual values by their key
print(student["name"])
print(student["age"])
print(student["year"])

Notice how we use square brackets with the key name inside — student["name"] — to get the value we want. This is different from lists, where you use a number index like my_list[0]. With dictionaries, you go directly to the data you need by name.

Example 2: Modifying a Dictionary

Dictionaries are mutable — you can change them after they’ve been created. You can add new key-value pairs, update existing values, and delete entries you no longer need.

# Start with a basic student dictionary
student = {"name": "Alice", "age": 15, "year": 10}
print("Original:", student)

# Add a new key-value pair
student["house"] = "Maple"
print("After adding house:", student)

# Update an existing value
student["age"] = 16
print("After birthday:", student)

# Delete a key-value pair
del student["year"]
print("After removing year:", student)

Adding and updating use exactly the same syntax: dictionary[key] = value. If the key already exists, the value is replaced. If the key is new, it gets added. The del keyword removes a key and its value entirely.

Example 3: Looping Through a Dictionary

Just like lists, you can loop through dictionaries. The most useful approach is to loop through both the keys and the values at the same time using the .items() method.

# A dictionary of student information
student = {"name": "Alice", "age": 15, "year": 10, "house": "Maple"}

# Loop through all key-value pairs
print("--- Student Record ---")
for key, value in student.items():
    print(f"{key}: {value}")

print()

# You can also loop through just the keys
print("Keys:", list(student.keys()))

# Or just the values
print("Values:", list(student.values()))

The .items() method gives you each key-value pair as a tuple, which we unpack into two variables: key and value. You can name these variables anything you like — k, v or field, data would work just as well — but clear names make your code easier to read.

Example 4: Nested Dictionaries

A dictionary can contain other dictionaries as its values. This is called nesting, and it is incredibly useful for organising complex, structured data — like a school database where each year group has its own set of details.

# A dictionary of dictionaries — each year group has student details
school = {
    "Year10": {"name": "Alice", "age": 15, "form": "10B"},
    "Year11": {"name": "Bob", "age": 16, "form": "11A"},
    "Year12": {"name": "Charlie", "age": 17, "form": "12C"}
}

# Access the whole Year10 dictionary
print("Year 10 record:", school["Year10"])

# Access a specific value inside a nested dictionary
print("Year 11 student:", school["Year11"]["name"])
print("Year 12 form group:", school["Year12"]["form"])

# Loop through all year groups
print("\n--- All Students ---")
for year, details in school.items():
    print(f"{year}: {details['name']} (Age {details['age']}, Form {details['form']})")

To access data inside a nested dictionary, you chain square brackets: school["Year11"]["name"] first looks up the "Year11" key (which gives you a dictionary), then looks up "name" inside that dictionary. Think of it as opening one box to find another box inside.

Example 5: Dictionary with List Values

Dictionary values don’t have to be single items — they can be lists. This is perfect when you need to store multiple values for a single key, such as a student’s test scores across several assessments.

# Each subject maps to a list of test scores
subjects = {
    "Maths": [85, 90, 78],
    "English": [72, 88, 91],
    "Science": [95, 82, 88]
}

# Access all Maths scores
print("Maths scores:", subjects["Maths"])

# Access a specific score (first English test)
print("First English test:", subjects["English"][0])

# Add a new score to Science
subjects["Science"].append(90)
print("Updated Science scores:", subjects["Science"])

# Calculate averages for each subject
print("\n--- Subject Averages ---")
for subject, scores in subjects.items():
    average = sum(scores) / len(scores)
    print(f"{subject}: {average:.1f}%")

Because the values are lists, you can use all the list operations you already know — .append(), indexing with [0], len(), sum(), and so on. The combination of dictionaries and lists is one of the most powerful patterns in Python.

Example 6: Building a Dictionary with a Loop

You don’t always have to type out every key-value pair by hand. You can build a dictionary dynamically using a loop — this is essential when working with data from files, user input, or calculations.

# Build a dictionary of squares using a loop
squares = {}
for number in range(1, 11):
    squares[number] = number ** 2

print("Squares:", squares)
print("7 squared is:", squares[7])

# Build a dictionary from two lists using zip()
names = ["Alice", "Bob", "Charlie", "Diana"]
scores = [85, 92, 78, 95]

student_scores = {}
for name, score in zip(names, scores):
    student_scores[name] = score

print("\nStudent Scores:", student_scores)

# Find the top student
top_student = max(student_scores, key=student_scores.get)
print(f"Top student: {top_student} with {student_scores[top_student]}%")

The pattern is simple: start with an empty dictionary {}, then inside a loop, assign values using dictionary[key] = value. The zip() function is particularly handy — it pairs up items from two lists so you can loop through them together.

Where Are Dictionaries Used in the Real World?

Dictionaries are not just a classroom concept — they are used everywhere in professional software development. Here are some of the most important real-world applications: