Apr 20, 2026

Dictionaries: Labels for Your Data

A list keeps data in order, and you grab items by position (scores[0]). But often you want to look things up by name: what’s Ada’s score? what’s the capital of France? For that, Python has a dictionary — it stores labels paired with values. Real datasets are full of labeled data, so this is a big one for Phase III.

Making a dictionary

Use curly braces { }, with each entry written as label: value (the label is called a key):

Your code — edit it, then press Run

scores = {"Ada": 90, "Linus": 85, "Grace": 100}
print(scores)

Think of it like a real dictionary: you look up a word (the key) to get its meaning (the value).

Looking things up

Use square brackets with the key (not a position) to get its value:

Your code — edit it, then press Run

scores = {"Ada": 90, "Linus": 85, "Grace": 100}
print("Ada scored", scores["Ada"])
print("Grace scored", scores["Grace"])

Try it 🎯

Make a dictionary of three countries and their capitals ("France": "Paris", …) and print one of them.

Adding and changing entries

Assign to a new key to add it; assign to an existing key to change it:

Your code — edit it, then press Run

scores = {"Ada": 90}
scores["Sam"] = 75      # add a new person
scores["Ada"] = 95      # change Ada's score
print(scores)

Checking and looping

in checks whether a key exists. And a for loop walks through the keys, so you can visit every entry:

Your code — edit it, then press Run

scores = {"Ada": 90, "Linus": 85, "Grace": 100}

print("Is Ada here?", "Ada" in scores)
print("Is Bob here?", "Bob" in scores)

for name in scores:
  print(name, "scored", scores[name])

The loop gives you each key (name), and scores[name] gets that person’s value. This is how you process a whole labeled dataset — exactly the shape of data you’ll meet in Phase III.

Predict it 🔮

What does this print? Look up the key carefully:

Your code — edit it, then press Run

prices = {"apple": 3, "banana": 2}
print(prices["banana"])

(It prints 2 — the value labeled "banana". With dictionaries you look up by the key’s name, not a number.)

Fix the bug 🐞

This crashes with a “KeyError” — it’s asking for a key that isn’t in the dictionary. Look at the spelling/capitals:

Your code — edit it, then press Run

ages = {"Ada": 36, "Linus": 54}
print(ages["ada"])

(Keys are exact: the dictionary has "Ada" (capital A), but the code asks for "ada". Match it: ages["Ada"].)

Your mission 🚀

Build a “pet age” dictionary: store three pets and their ages, add one more with assignment, then loop through and print each pet with its age:

Your code — edit it, then press Run

pets = {"Rex": 3, "Whiskers": 5, "Bubbles": 1}
# add one more pet, then loop and print each pet and its age

What you learned today

A dictionary stores key: value pairs in { } — labels for your data.
Look up by key: scores["Ada"]; add or change with scores["Sam"] = 75.
key in dict checks for a key; for key in dict: loops through them.
Lists are ordered by position; dictionaries are labeled by key. You’ll use both constantly with real data.

Next time we level up functions — making them return an answer you can use, not just print it. 🎁

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