Nested collections

2 min read 5 runnable examples Quiz included

Real data is rarely flat. A list of students, each with several grades; a shop with categories, each with products. You model this by putting collections inside other collections.

A list of dictionaries

The most common shape: many records, each with named fields. It looks just like JSON from a web API.

Python

students = [
    {"name": "Asha", "grade": 92, "city": "Mumbai"},
    {"name": "Leo", "grade": 78, "city": "London"},
    {"name": "Mei", "grade": 85, "city": "Osaka"},
]

for s in students:
    print(f"{s['name']} from {s['city']}: {s['grade']}")

top = max(students, key=lambda s: s["grade"])
print("Top student:", top["name"])

A dictionary of lists

Python

courses = {
    "Python": ["Asha", "Leo"],
    "SQL": ["Mei"],
}
courses["SQL"].append("Raj")
courses.setdefault("Java", []).append("Asha")
print(courses)

for course, people in courses.items():
    print(course, "has", len(people), "learner(s)")

A grid: a list of lists

A two-dimensional grid is a list of rows, each row a list:

Python

board = [
    ["X", "O", "X"],
    [" ", "X", "O"],
    ["O", " ", "X"],
]
print(board[1][2])          # row 1, column 2
board[1][0] = "O"

for row in board:
    print(" | ".join(row))

Reaching deep inside

Chain the lookups one level at a time, from the outside in:

Python

shop = {
    "name": "CodeMIRAI Store",
    "categories": [
        {"title": "Books", "items": [{"name": "Python 101", "price": 25}]},
        {"title": "Mugs", "items": [{"name": "Hello World mug", "price": 12}]},
    ],
}
print(shop["categories"][0]["items"][0]["name"])

for cat in shop["categories"]:
    for item in cat["items"]:
        print(f"{cat['title']}: {item['name']} costs {item['price']}")

Python

wrong = [[0] * 3] * 3
wrong[0][0] = 1
print(wrong)

right = [[0] * 3 for _ in range(3)]
right[0][0] = 1
print(right)

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