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

Dictionaries and Sets — Lookup-Based Collections

prices["mango"] doesn't return None when the key is missing. It stops your program with a KeyError — on purpose.

Hiring signal: Reaches for .get() with an explicit default instead of guessing that a missing key silently returns None

What you will learn

Create, access, and update dictionaries; use .get() to safely handle missing keys
Explain why a missing dictionary key raises KeyError instead of returning None
Use a set for uniqueness and fast membership testing
Predict whether a given .get() call returns a real value or a default

Introduction

Type: Learn Languages: Python Prerequisites: Lesson 01 (Lists and Tuples) Time: ~35 minutes

Objective

Learning objectives

Create, access, and update dictionaries; use .get() to safely handle missing keys
Explain why a missing dictionary key raises KeyError instead of returning None
Use a set for uniqueness and fast membership testing
Predict whether a given .get() call returns a real value or a default

What you're building

A script (word_counter.py) that:

Takes a real short block of text (at least 3 sentences, hardcoded as a string is fine)
Builds a dictionary counting how many times each word appears, using the if word in counts pattern
Rewrites the same counting logic a second time using .get(word, 0) + 1 instead, producing an identical result
Prints the 3 most frequent words, using sorted(counts.items(), key=..., reverse=True)
Separately, builds a set of all unique words and prints how many total words vs. unique words there were

scores = {"Ada": 92, "Grace": 0}. What does scores.get("Grace", "no score") return?

This is the exact trap from this lesson's inventory example. .get()'s fallback only triggers when the key doesn't exist in the dictionary at all. "Grace" absolutely exists as a key here — its value just happens to be 0, which is a completely valid, real value. .get() has no concept of "this value looks empty," only "this key does or doesn't exist."

Unlock the full lesson

You've read the first 2 sections. The rest of this lesson covers The Problem, Check Yourself, Key Terms & Next — plus a hands-on lab, quiz, and project artifact.

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