PCC·06 · Chapter record

Dictionaries

Chapter 6 — storing and connecting related information

Done

Five sessions on Python's other core data structure: building a dictionary, looping through it three different ways, checking membership, and nesting lists and dictionaries inside each other. Each one explains the concept, lists the key terms, then gives you exercises — try them yourself before revealing the code.

6.1

A Simple Dictionary

Concept

A dictionary is a collection of key-value pairs, wrapped in braces {} — each key connects to a value, separated by a colon.
Access a value with dict[key]; add or modify one with dict[key] = value — a dictionary can start empty ({}) and grow one pair at a time.
del dict[key] removes a key-value pair permanently.
Asking for a key that doesn't exist raises a KeyError — .get(key, default) is the safe alternative, returning your default instead of crashing.

Key Terms

dictionary: a collection of key-value pairs, written with { }
key-value pair: a key connected to its value by a colon
KeyError: raised when you ask for a key that doesn't exist
.get(key, default): safely reads a value, returning default if the key is missing

Exercise

Activity 1

Store a person's first name, last name, age, and city in a dictionary, then print each piece of information.

Activity 2

Build a small glossary: use five programming terms as keys and their meanings as values, then print each term and meaning.

6.2

Looping Through a Dictionary

Concept

for key, value in d.items(): hands you both the key and the value together, one pair per iteration.
Loop over d.keys() (or just for x in d: — looping a dictionary directly defaults to its keys) when you only need the keys.
Loop over d.values() when you only need the values — wrap it in set() to drop duplicates.
Wrap sorted() around .keys() to loop in alphabetical order instead of insertion order.

Key Terms

.items(): loop target yielding (key, value) pairs
.keys() / .values(): loop targets yielding just the keys, or just the values
set(): a collection of unique items — handy for de-duplicating values

Exercise

Activity 3

Make a dictionary of three rivers and the country each runs through, then use a loop to print a sentence about each one.

Activity 4

Using that same dictionary, loop through just the river names, then separately loop through just the countries.

6.3

Checking Membership

Concept

Use in or not in with .keys() to check whether something is already in a dictionary — 'erin' not in favorite_languages.keys().
Combine this with a separate list to give some entries special treatment while looping — like printing a bonus message only for names that are also in a "friends" list.
This scales to a dictionary with a million entries exactly as easily as one with five.

Key Terms

in / not in (with .keys()): checks whether a key exists in a dictionary

Exercise

Activity 5

Make a list of people who should take a poll, some already in your dictionary and some not. Loop through it, thanking those who've responded and inviting those who haven't.

6.4

Nesting: Lists & Dictionaries

Concept

A list of dictionaries lets you manage many similar objects — each alien is its own {} inside one [] list.
A list inside a dictionary lets one key hold multiple values — 'toppings': ['mushrooms', 'extra cheese'].
Nest a for loop inside a for loop to walk through each dictionary entry's list value in turn.

Key Terms

list of dictionaries: a [] whose elements are each a {}
list value: a dictionary value that is itself a [] of items

Exercise

Activity 6

Make three small dictionaries representing pets (each with a kind and an owner), store them in a list, then loop through and print a sentence about each pet.

Activity 7

Store a pizza order with a crust and a list of toppings. Print a summary sentence, then loop through the toppings list to print each one.

6.5

A Dictionary in a Dictionary

Concept

A dictionary can hold another dictionary as a value — ideal for many users, each with their own set of details.
Loop the outer dictionary with .items(); for each username you get an inner dictionary you can index or loop again.
Keep nesting shallow — if you find yourself several levels deep, there's usually a simpler structure.

Key Terms

dictionary of dictionaries: a {} whose values are themselves {}

Exercise

Final challenge

Build a dictionary of at least two usernames, each mapping to an inner dictionary of first name, last name, and location. Loop through it and print a formatted profile for each user.