Dictionaries

What you'll learn

Store data as pairs of key and value
Look up, change and add values
Go through all pairs with items()

Look things up instead of counting through

In a list you find values by their position. Often it’s more practical to find a value by a name – like in a dictionary where you look up a word. That’s exactly what a dictionary (type dict) does:

phone_book = {"Mia": "0171 1234", "Ben": "0160 9876"}
print(phone_book["Mia"])

A dictionary consists of pairs: key: value. The key goes before the colon, the value after it. Everything together goes into curly brackets { }.

Changing and adding

With the same notation you change values – or create new pairs if the key doesn’t exist yet. Use Steps to watch the dictionary grow:

player = {"name": "Lena", "level": 3}
player["level"] = 4
player["lives"] = 3
print(player)
print(len(player))

Looking up safely

If you ask for a key that doesn’t exist, the program crashes with a KeyError. Check first with in, or use get() with a fallback value:

grades = {"Maths": 2, "English": 1}

if "Art" in grades:
    print(grades["Art"])
else:
    print("No grade for Art.")

print(grades.get("Sport", "still open"))

Going through all pairs

With .items() you get the key and the value in a loop each time:

scores = {"Team Red": 12, "Team Blue": 15, "Team Green": 9}

for team, score in scores.items():
    print(f"{team} has {score} points")

best = max(scores, key=scores.get)
print(f"{best} is in the lead!")

Remember: Every key appears only once in a dictionary. If you write a key in a second time, the old value is overwritten.

Quick check

Before you start the task: what does this program print? Type your guess – then Python works it out.

ages = {"Ana": 12, "Ben": 13}
ages["Ana"] = ages["Ana"] + 1
print(ages)

Your task

Your turn

The dictionary stock shows how much fruit a shop has. Add bananas with 7 pieces and increase the apples by 3. Then print every fruit that is still in stock (more than 0) on its own line, e.g. apples: 8. Fruit with 0 pieces should not appear.

stock = {"apples": 5, "pears": 0, "kiwis": 12}

# Change the stock and print it:

Something to think about
You create a new entry like this – stock["bananas"] = 7
Which command helps?
Read the old value and add 3 – stock["apples"] = stock["apples"] + 3
Go through the dictionary with for fruit, count in stock.items() and print each one as “name: count”.
Pears have 0 pieces and should not appear. Check with if count > 0.

Gap code – fill in the ___

                        stock = {"apples": 5, "pears": 0, "kiwis": 12}

stock["bananas"] = ___
stock["apples"] = ___

for fruit, count in ___:
    if ___:
        print(___)

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

stock = {"apples": 5, "pears": 0, "kiwis": 12}

stock["bananas"] = 7
stock["apples"] = stock["apples"] + 3

for fruit, count in stock.items():
    if count > 0:
        print(f"{fruit}: {count}")

Practise more in the training roomEndless new tasks on this topic – with XP and streaks.