Lesson 07 · Dictionaries¶

What is a dictionary?¶

A dictionary stores key → value pairs, like a real dictionary where the word is the key and the definition is the value:

student = {
    "name": "Andrew",
    "age": 14,
    "average": 9.5,
    "is_honor_student": True
}

Unlike lists (where you access elements via a numeric index), dictionaries are accessed via keys:

print(student["name"])      # Andrew
print(student["average"])   # 9.5

Creating a dictionary¶

# Empty dictionary
data = {}

# Dictionary with initial values
car = {
    "brand": "Dacia",
    "model": "Logan",
    "year": 2020,
    "km": 45000
}

Keys can be str, int, or float. Values can be any type.

Accessing values¶

With []¶

print(car["brand"])   # Dacia

If the key doesn't exist, you get a KeyError.

With .get() — safer¶

print(car.get("color"))                  # None (no error)
print(car.get("color", "unknown"))       # unknown

Adding and modifying¶

car["color"] = "white"   # add a new key
car["km"] = 46000         # modify an existing value
print(car)

Deleting keys¶

del car["km"]              # delete the key
year = car.pop("year", None)   # delete and return the value

Checking key existence¶

if "brand" in car:
    print("We have brand:", car["brand"])

The main methods¶

person = {"name": "Helen", "age": 16, "city": "Cluj"}

print(person.keys())    # dict_keys(['name', 'age', 'city'])
print(person.values())  # dict_values(['Helen', 16, 'Cluj'])
print(person.items())   # dict_items([('name', 'Helen'), ...])
print(len(person))      # 3

Iterating¶

Just the keys¶

for key in person:
    print(key)
# name
# age
# city

Keys and values with .items()¶

for key, value in person.items():
    print(f"{key}: {value}")
# name: Helen
# age: 16
# city: Cluj

Nested dictionaries¶

classroom = {
    "Andrew": {"romanian_grade": 9, "math_grade": 8},
    "Mary": {"romanian_grade": 10, "math_grade": 9},
}

print(classroom["Mary"]["math_grade"])   # 9

Exercises¶

Exercise 1 — The capital¶

Create a dictionary with 5 country-capital pairs. Ask the user for a country and display the capital (or "Unknown" if it doesn't exist).

capitals = {
    "Romania": "Bucharest",
    "France": "Paris",
    "Germany": "Berlin",
    "Italy": "Rome",
    "Spain": "Madrid"
}
country = input("Country: ")
print(capitals.get(country, "Unknown"))

Exercise 2 — Counting occurrences¶

Count how many times each letter appears in the word "programming".

word = "programming"
frequency = {}
for letter in word:
    if letter in frequency:
        frequency[letter] += 1
    else:
        frequency[letter] = 1
for letter, count in frequency.items():
    print(f"{letter}: {count}")

Exercise 3 — Update¶

Starting from {"apples": 5, "pears": 3}, add "cherries": 10, change apples to 8, and delete pears.

basket = {"apples": 5, "pears": 3}
basket["cherries"] = 10
basket["apples"] = 8
del basket["pears"]
print(basket)   # {'apples': 8, 'cherries': 10}

Mini-project: Contact book¶

Build a small contact-management program that lets you add, search, and list contacts.

Example run:

1. Add contact
2. Search contact
3. List all
4. Exit
Your choice: 1
Name: Anna
Phone: 0722123456
Contact added!

contacts = {}

while True:
    print("\n1. Add contact")
    print("2. Search contact")
    print("3. List all")
    print("4. Exit")
    choice = input("Your choice: ")

    if choice == "1":
        name = input("Name: ")
        phone = input("Phone: ")
        contacts[name] = phone
        print("Contact added!")
    elif choice == "2":
        name = input("Search by name: ")
        print(contacts.get(name, "Contact not found."))
    elif choice == "3":
        if contacts:
            for name, phone in contacts.items():
                print(f"{name}: {phone}")
        else:
            print("No contacts saved.")
    elif choice == "4":
        print("Goodbye!")
        break
    else:
        print("Invalid option.")

Summary¶

• Dictionaries store key-value pairs: {"key": value}
• Access: dict["key"] or dict.get("key", default)
• Add/modify: dict["key"] = value
• Delete: del dict["key"] or dict.pop("key")
• Iterate with .items() for keys and values together
• Check: "key" in dict

Next step: → Lesson 08: Functions