Functions and Dictionaries

Functions and dictionaries go well together. A function can take a dictionary as an argument and use the values it needs:

def describe(pet):
    return f"{pet['name']} is a {pet['species']}."

rex = {"name": "Rex", "species": "dog", "age": 3}
print(describe(rex))
print(describe({"name": "Kiwi", "species": "parrot", "age": 4}))

Rex is a dog.
Kiwi is a parrot.

The parameter pet is a name for whatever dictionary you pass in the call: in the first call it's Rex's dictionary, in the second one Kiwi's. Instead of separate arguments for the name and the species, you pass one dictionary, and the function takes what it needs from it.

A function can also take a list of dictionaries. This one returns the dictionary of the oldest pet. It uses the pattern for the largest value from the lesson on loops:

def oldest(pets):
    result = pets[0]
    for pet in pets:
        if pet["age"] > result["age"]:
            result = pet
    return result

pets = [
    {"name": "Rex", "species": "dog", "age": 3},
    {"name": "Bella", "species": "cat", "age": 5},
    {"name": "Kiwi", "species": "parrot", "age": 4}
]
winner = oldest(pets)
print(winner["name"], "is the oldest.")

Bella is the oldest.

And a function can return a new dictionary. That's handy when you create many dictionaries of the same shape: you write the keys only once, in the function, and every dictionary it returns has the same keys:

def create_pet(name, species):
    return {"name": name, "species": species, "age": 0}

new_pets = []
new_pets.append(create_pet("Coco", "rabbit"))
new_pets.append(create_pet("Max", "dog"))
print(new_pets)

[{'name': 'Coco', 'species': 'rabbit', 'age': 0}, {'name': 'Max', 'species': 'dog', 'age': 0}]

"name": name may look strange. On the left of the colon is the key, the text "name" in quotes. On the right is its value: the parameter name, which holds "Coco" in the first call. A new pet is always 0 years old, so "age" is 0 in every dictionary the function creates.

A typical mistake is to forget return. The function then gives back None, and the list would be [None, None].