Phase 2: Programming Fundamentals II · ~35 minutes · Python · uv

Lists and Tuples — Ordered Collections

list[1:3] stops before index 3, not at it. Every slicing bug you'll ever write traces back to forgetting that one fact.

Hiring signal: Chooses list vs. tuple deliberately, and reasons correctly about slice boundaries instead of guessing and checking

What you will learn

Create, index, slice, and mutate lists; explain why tuples can't be mutated the same way
Explain why slicing is exclusive on the end index and use that rule to avoid off-by-one bugs
Diagnose a real IndexError by tracing it back to a boundary miscalculation
Decide when immutability (a tuple) is the right choice over a list

Introduction

Type: Learn Languages: Python Prerequisites: Phase 01 (Programming Fundamentals I) Time: ~35 minutes

Objective

Learning objectives

Create, index, slice, and mutate lists; explain why tuples can't be mutated the same way
Explain why slicing is exclusive on the end index and use that rule to avoid off-by-one bugs
Diagnose a real IndexError by tracing it back to a boundary miscalculation
Decide when immutability (a tuple) is the right choice over a list

What you're building

A script (student_records.py) using this data:

students = [
    {"name": "Ada", "grade": 92},
    {"name": "Grace", "grade": 88},
    {"name": "Alan", "grade": 95},
    {"name": "Katherine", "grade": 90},
    {"name": "Edsger", "grade": 78},
]

That:

Filters this list to only students with a grade of 90 or above
Slices out the last two students in the original list (using a negative, open-ended slice, not hardcoded positions)
Reorders the original list so it's sorted by grade, highest first (sorted(..., key=..., reverse=True))
Contains a comment demonstrating the students[len(students)] off-by-one mistake and its fix

Given numbers = [10, 20, 30, 40, 50], what does numbers[-3:-1] return?

Negative indices count from the end: -3 is 30, -2 is 40, -1 is 50. The slice numbers[-3:-1] starts at -3 (30) and stops before -1 (50) — so it includes -3 and -2, giving [30, 40]. The "stop is never included" rule from this lesson applies identically whether the indices are positive or negative, and mixing two negative indices in one slice is completely valid Python.

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