Dictionaries

Big idea

A dictionary (dict) stores key → value pairs. Instead of looking things up by position, you look them up by name.

Making a dictionary

ages = {"Ada": 36, "Grace": 85, "Linus": 28}

Each entry is key: value. Keys are usually strings or numbers and must be unique; values can be anything.

ages = {"Ada": 36, "Grace": 85}
print(ages["Ada"])
ages["Ada"] = 37
ages["Linus"] = 28
print(ages)
print(len(ages))

output
36
{'Ada': 37, 'Grace': 85, 'Linus': 28}
3

The same syntax, d[key] = value, adds a new key or replaces an existing one's value.

Missing keys

Asking for a key that isn't there is a KeyError. Two safe ways to ask:

stock = {"apple": 5, "pear": 0}
print("banana" in stock)
print(stock.get("banana"))
print(stock.get("banana", 0))
print(stock.get("apple", 0))

False
None
0
5

• key in d — checks the keys.
• d.get(key, default) — the value, or default if the key is missing.

Looping

prices = {"tea": 2.5, "cake": 3.0, "soup": 4.75}
for item in prices:
    print(item)
for item, price in prices.items():
    print(f"{item}: ${price:.2f}")
print(sum(prices.values()))

output
tea
cake
soup
tea: $2.50
cake: $3.00
soup: $4.75
10.25

Looping over a dict gives its keys, in the order they were added. .items() gives (key, value) pairs; .values() gives just the values.

Counting with a dict

A dict makes a perfect tally:

basket = ["apple", "pear", "apple", "fig", "apple"]
counts = {}
for fruit in basket:
    counts[fruit] += 1
print(counts)

output
{'apple': 3, 'pear': 1, 'fig': 1}

The first time a fruit appears it isn't a key yet, so counts[fruit] += 1 is a KeyError. Use counts[fruit] = counts.get(fruit, 0) + 1.

Removing

del ages["Linus"]        # remove a key (KeyError if missing)
age = ages.pop("Grace")  # remove and return its value

Watch it

Think of it like

A real dictionary: you don't read it page by page — you go straight to the word (the key) and read its meaning (the value).

Key terms

• Dictionary (dict) — A collection of key → value pairs.
• Key — The unique name you look up by.
• KeyError — Asking for a key that doesn't exist.