Nested data

Lists of dictionaries

Real data is rarely flat. The shape you will meet most often is a list of dictionaries — exactly what an API hands back:

people = [
    {"name": "Ada", "age": 36},
    {"name": "Alan", "age": 41},
]

people[0]["name"]     # "Ada"

Read the subscripts left to right: people[0] is a dictionary, and ["name"] looks inside it.

Dictionaries nest too:

config = {"server": {"host": "localhost", "port": 8080}}
config["server"]["port"]    # 8080

Your turn: write names_over(people, age) returning the names of everyone older than the given age, in the order they appear.

Grouping

Turning a flat list into a grouped dictionary is a task you will meet again and again:

groups = {}
for item in items:
    key = item["category"]
    if key not in groups:
        groups[key] = []
    groups[key].append(item["name"])

.setdefault(key, []) collapses the middle three lines into one — it returns the existing list, or inserts and returns a new one.

Your turn: write group_by_city(people) returning a dictionary mapping each city to the list of names living there, in the order they appear.