Dictionaries

Values with names

A list keeps values in order, and you find them by position. A dictionary keeps values with keys, and you find a value by its key. Each key with its value is a pair.

pet = {
    "name": "Biscuit",
    "age": 4,
    "is_friendly": True,
}

Reading and changing

Use square brackets with the key.

print(pet["name"])      # Biscuit
pet["age"] = 5          # change a value
pet["color"] = "brown"  # add a new pair

If you ask for a key that does not exist, Python stops with a KeyError. The get method is safer. It gives None, or a default value that you choose.

print(pet.get("weight"))            # None
print(pet.get("weight", "unknown")) # unknown

Useful tools

• "name" in pet is True when the key exists.
• pet.keys() gives the keys, pet.values() gives the values, and pet.items() gives the pairs.
• len(pet) is the number of pairs.

for key, value in pet.items():
    print(key, value)

Dictionaries inside lists

Most real data is a list of dictionaries. For example, a list of books, users, or products.

pets = [
    {"name": "Biscuit", "age": 4},
    {"name": "Pepper", "age": 2},
]

for pet in pets:
    print(pet["name"], "is", pet["age"])

Your turn

The dictionary book is already in the editor. Write your code under it. The tasks build on each other, so do them in order.