Nested Structures

Unit ID: M03-U06 Estimated active time: 25-35 minutes

Records often live inside sequences

Consider a small fictional dataset:

courses = [
    {"code": "AI-01", "title": "AI Foundations", "hours": 8},
    {"code": "PY-01", "title": "Python Foundations", "hours": 32},
]

The outer object is a list. Each list item is a dictionary. The access path must follow that shape.

Read one field step by step

first_course = courses[0]
first_title = first_course["title"]
print(first_title)

You can write the path directly:

print(courses[0]["title"])

The step-by-step version is easier to inspect while learning or debugging.

Add a nested collection

courses[0]["tags"] = ["ai", "beginner"]
print(courses[0]["tags"][1])

The access path means:

1. take list item 0;
2. read its "tags" field;
3. take tag item 1.

Draw the shape before guessing

Use indentation to make nesting visible:

courses: list
  item 0: dictionary
    code: string
    title: string
    hours: integer
    tags: list
      item 0: string
      item 1: string

When an access fails, check each step's type.

Common path errors

courses["title"]

This fails because courses is a list, not a dictionary.

courses[0][0]

This fails because the first record is a dictionary whose meaningful access uses keys.

courses[3]["title"]

This fails because the list has only two items.

Preserve the source before editing

To experiment with the second record:

working_course = courses[1].copy()
working_course["hours"] = 30

The original second dictionary remains unchanged because the edited fields are scalar values. If a nested list is edited, copy that nested list too.

Practice

Add a third fictional course dictionary. Then retrieve its code, add a two-item tag list, and retrieve the final tag. Explain each access step and its type.

Takeaway

Nested access follows the data shape one level at a time. Separate steps while debugging, inspect each type, and copy the level you intend to change. Next, we will summarise and convert collections using readable built-in operations.