A list is an ordered, mutable sequence.

xs = [30, 10, 20]
xs.append(40)
print(xs[0], xs[-1])  # 30 40
print(xs[1:3])        # [10, 20]
print(xs[::-1])       # reversed copy

Slices never raise for out-of-range bounds. They return whatever exists, and they build a new list.

• xs.sort() changes the list and returns None.
• sorted(xs) returns a new list and leaves xs alone.
• Both accept reverse=True and key=len style functions.

Do not change a list passed to you unless that is the function's stated job. The caller may still need it.

A comprehension builds a list from a loop in one expression:

nums = [1, 2, 3, 4]
big = [n * 10 for n in nums if n > 2]
print(big)  # [30, 40]

Write top_three(scores) returning the three largest scores, highest first, without changing the input. Write chunks(items, size) returning a list of consecutive slices of at most size items.

The explanation opens here once every check passes.