Dictionaries

A list finds items by position: scores[0], scores[1]. Often it's much more natural to find things by name: a word's definition, a user's email, a product's price. A dictionary stores key-value pairs, so you look up a value using its key.

Creating a dictionary

Write key: value pairs inside curly braces:

person = {
    "name": "Ada",
    "age": 36,
    "city": "London",
}

print(person)
print(type(person))
print(len(person))   # number of pairs

Keys are usually strings, but they can be numbers or tuples too. Each key appears only once in a dictionary. Values can be anything, including lists or other dictionaries.

Looking up values

Put the key in square brackets:

person = {"name": "Ada", "age": 36, "city": "London"}

print(person["name"])
print(person["age"])

Asking for a key that doesn't exist raises a KeyError:

person = {"name": "Ada", "age": 36}
print(person["email"])

Safer lookups with get()

get() returns None instead of crashing, or a default value that you choose:

person = {"name": "Ada", "age": 36}

print(person.get("name"))
print(person.get("email"))
print(person.get("email", "no email"))

Use in to check whether a key exists:

person = {"name": "Ada", "age": 36}

print("age" in person)
print("email" in person)

Adding and changing entries

Assigning to a key adds it if it's new, or replaces its value if it already exists:

stock = {"apples": 10, "pears": 4}

stock["bananas"] = 7     # add
stock["apples"] = 12     # update
stock["pears"] += 1      # update based on the old value

print(stock)

update() adds or changes several entries at once:

stock = {"apples": 10}
stock.update({"pears": 3, "apples": 15})
print(stock)

defaults = {"theme": "light", "font_size": 14}
choices = {"font_size": 18}

settings = defaults | choices
print(settings)
print(defaults)    # unchanged

Removing entries

stock = {"apples": 10, "pears": 4, "bananas": 7, "kiwis": 2}

removed = stock.pop("pears")    # remove and return the value
print(removed, stock)

del stock["bananas"]            # remove
print(stock)

stock.clear()                   # remove everything
print(stock)

Keys, values and items

Three methods give you views of the dictionary's contents:

prices = {"coffee": 3.5, "tea": 2.8, "juice": 4.0}

print(list(prices.keys()))
print(list(prices.values()))
print(list(prices.items()))   # list of (key, value) tuples

These come in handy for calculations:

prices = {"coffee": 3.5, "tea": 2.8, "juice": 4.0}

print(sum(prices.values()))
print(max(prices.values()))
print(sorted(prices))         # sorted keys

You'll loop over these in the For Loops lesson, which is where dictionaries really shine.

Counting with a dictionary

A very common pattern is counting things. get(key, 0) gives 0 the first time a key is seen:

votes = {}

votes["red"] = votes.get("red", 0) + 1
votes["blue"] = votes.get("blue", 0) + 1
votes["red"] = votes.get("red", 0) + 1

print(votes)

from collections import Counter

votes = ["red", "blue", "red", "green", "red", "blue"]
counts = Counter(votes)

print(counts)
print(counts["red"])
print(counts["purple"])        # missing keys count as 0, no KeyError
print(counts.most_common(2))   # the two most common, as (value, count) pairs

student = {
    "name": "Mia",
    "grades": [88, 92, 79],
    "address": {
        "city": "Leeds",
        "postcode": "LS1 4AP",
    },
}

print(student["grades"][1])
print(student["address"]["city"])

student["grades"].append(95)
print(student["grades"])

distances = {("London", "Paris"): 344}
print(distances[("London", "Paris")])

bad = {["a", "b"]: 1}

Exercises

Exercise 1: Contact card

Create a dictionary for a contact with keys name, phone and email, then print the phone number.

contact = {}

Exercise 2: Update the inventory

Sell 3 apples, add a new item "mangos" with 6 in stock, and remove "pears". It should print {'apples': 7, 'bananas': 5, 'mangos': 6}.

inventory = {"apples": 10, "pears": 4, "bananas": 5}

print(inventory)

Exercise 3: Safe lookup

Look up the capital of country. If the country isn't in the dictionary, print Unknown instead of crashing.

capitals = {"France": "Paris", "Japan": "Tokyo", "Kenya": "Nairobi"}
country = "Brazil"

Exercise 4: Order total

The order dictionary maps items to quantities, and prices maps items to prices. Calculate the total for coffee and cake and print Total: 12.0.

prices = {"coffee": 3.0, "cake": 4.5, "tea": 2.5}
order = {"coffee": 1, "cake": 2}

Dictionary methods at a glance

Each example starts from d = {"a": 1, "b": 2}. The Result column shows what the method gives back, or what d looks like afterwards for methods that change it.

Useful operations that aren't methods:

Summary

• A dictionary maps keys to values: {"name": "Ada"}.
• Look up values with d[key]. A missing key raises KeyError. Use d.get(key, default) to be safe.
• d[key] = value adds or updates. pop() and del remove.
• keys(), values() and items() give you the contents.