Python Dictionaries

A list is great when items are numbered: first, second, third. But often you want to find data by a name, not a number. Python dictionaries let you do exactly that by storing key-value pairs. In this lesson, you'll learn how to create dictionaries, read, add, change and delete values, loop through them, and nest them.

What is a dictionary in Python?

A dictionary (type dict) is a collection of key-value pairs. The key is the label you search by, and the value is the data attached to it. You write a dictionary with curly braces, a colon between each key and its value, and commas between pairs:

contacts = {
    "Mom": "+1 555 0100",
    "Alex": "+44 20 7946 0018",
    "Pizza place": "+1 555 0199",
}

print(contacts["Alex"])
print(len(contacts))

+44 20 7946 0018
3

len() counts the pairs, not the individual keys and values.

How do you create a dictionary?

Besides curly braces, you can use the built-in dict() function with key=value arguments. An empty pair of braces makes an empty dictionary:

product = {"name": "Desk lamp", "price": 24.99, "in_stock": True}
settings = dict(theme="dark", volume=7)
cart = {}

print(product)
print(settings)
print(cart)

{'name': 'Desk lamp', 'price': 24.99, 'in_stock': True}
{'theme': 'dark', 'volume': 7}
{}

Values can be any type: strings, numbers, booleans, lists, even other dictionaries.

Accessing values: [] vs get()

There are two ways to read a value. Square brackets are short, but they raise a KeyError if the key doesn't exist:

profile = {"username": "skyrunner", "country": "Canada"}
print(profile["username"])
print(profile["email"])

skyrunner
KeyError: 'email'

The get() method is the safe option. It returns None for a missing key, or a default value that you choose:

profile = {"username": "skyrunner", "country": "Canada"}
print(profile.get("country"))
print(profile.get("email"))
print(profile.get("email", "not provided"))

Canada
None
not provided

Adding and updating items

Assigning to a key does double duty: if the key is new, the pair is added; if it already exists, its value is replaced. That's because keys are unique.

stock = {"apples": 12, "bananas": 6}

stock["cherries"] = 30   # new key: added
stock["apples"] = 10     # existing key: updated
stock["bananas"] += 4    # change using the current value

print(stock)

{'apples': 10, 'bananas': 10, 'cherries': 30}

To change several pairs at once, use update(). You'll see it and many more in Python dictionary methods.

Deleting items: del and pop()

del removes a pair by key. pop() removes it and gives you the value back, which is handy when you still need it:

order = {"burger": 8.5, "fries": 3.0, "soda": 2.0}

del order["soda"]
removed_price = order.pop("fries")

print(order)
print("Refund:", removed_price)

{'burger': 8.5}
Refund: 3.0

Both raise a KeyError if the key isn't there, unless you give pop() a default, like order.pop("salad", 0).

How do you loop through a dictionary?

A plain for loop over a dictionary gives you its keys. Use values() for the values, and items() for both at once:

scores = {"Lina": 88, "Omar": 95, "Kate": 79}

for name in scores:
    print(name)

for points in scores.values():
    print(points)

for name, points in scores.items():
    print(f"{name} scored {points}")

Lina
Omar
Kate
88
95
79
Lina scored 88
Omar scored 95
Kate scored 79

items() hands back each pair as a tuple, and name, points unpacks it into two variables, the same way you unpacked tuples.

Checking whether a key exists

The in operator checks keys, not values. Like sets, dictionaries use hashing, so this check is very fast even for huge dictionaries.

capitals = {"France": "Paris", "Japan": "Tokyo"}
print("Japan" in capitals)
print("Tokyo" in capitals)
print("Tokyo" in capitals.values())

True
False
True

Nested dictionaries

A value can be another dictionary. This is perfect for records with several fields, like a product catalog. Chain the keys to dig in:

catalog = {
    "SKU101": {"name": "Running shoes", "price": 79.99, "sizes": [8, 9, 10]},
    "SKU102": {"name": "Water bottle", "price": 12.50, "sizes": []},
}

print(catalog["SKU101"]["name"])
print(catalog["SKU101"]["sizes"][-1])

for sku, info in catalog.items():
    print(f"{sku}: {info['name']} costs ${info['price']}")

Running shoes
10
SKU101: Running shoes costs $79.99
SKU102: Water bottle costs $12.5

Notice the f-string uses single quotes inside (info['name']) because the string itself uses double quotes. See f-strings for more.

What can be a dictionary key?

Keys must be immutable (unchangeable), such as strings, numbers, booleans or tuples. Lists can't be keys because they can change:

flights = {("JFK", "LHR"): "7h 05m"}   # a tuple key works
print(flights[("JFK", "LHR")])

routes = {["JFK", "LHR"]: "7h 05m"}

7h 05m
TypeError: unhashable type: 'list'

Values have no such rule. A value can be anything, including a list.

Real-world example: a user profile

user = {
    "name": "Priya",
    "plan": "free",
    "playlists": ["Workout", "Chill"],
}

user["plan"] = "premium"
user["playlists"].append("Road trip")
user["verified"] = True

print(f"{user['name']} is on the {user['plan']} plan")
print("Playlists:", len(user["playlists"]))
print("Verified:", user.get("verified", False))

Priya is on the premium plan
Playlists: 3
Verified: True

Common mistakes with dictionaries

1. Reading a key that doesn't exist

profile["email"] crashes with KeyError when the key is missing. Use profile.get("email") or check with in first.

2. Expecting duplicate keys to both be kept

prices = {"tea": 2.5, "coffee": 3.0, "tea": 2.75}
print(prices)

{'tea': 2.75, 'coffee': 3.0}

A key can appear only once. The last value wins.

3. Accessing by position

menu = {"pasta": 11, "salad": 8}
print(menu[0])

KeyError: 0

Square brackets take a key, not an index. Python looks for a key named 0 and doesn't find one.

Practice exercise

weather = {"city": "Sydney", "temp_c": 24, "condition": "sunny"}

weather["temp_c"] = 26
weather["humidity"] = 60

for key, value in weather.items():
    print(f"{key}: {value}")

city: Sydney
temp_c: 26
condition: sunny
humidity: 60

What's next?

You now know how to build and use dictionaries. Next, explore the full toolbox in Python dictionary methods, including setdefault(), update(), fromkeys() and dictionary comprehensions.