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Workshop 9 — Homework

Welcome to your ninth homework! Last week you built your first classes. This week we make those objects do much more: print nicely (__str__ / __repr__), compare with == (__eq__), share data across every object (class attributes), reuse each other's code (inheritance and super()), and expose computed values as if they were plain attributes (@property). Several tasks build on last week's classes — but now your objects are far more powerful.

Deadline: before Workshop 10.

How to submit: push everything to the same GitHub repository you used for the previous homeworks, then re-send the repository link to your instructor.

Put this homework in its own folder so it does not mix with the earlier ones:

cd python-homework        # the repo from Homework 1
mkdir workshop_9_homework
cd workshop_9_homework
# ...create your exercise files in here...

When you are done, commit and push from the repository folder:

git add .
git commit -m "Add workshop 9 homework"
git push

If a command fails, read the error message carefully — it usually tells you what is wrong. If you are still stuck, take a screenshot and bring it to the next workshop.

Python exercises

Create the files below and write Python code to solve each task. Use everything we have covered so far: variables, f-strings, input(), int() / float(), if / elif / else, and / or / not, while, for + range(), +=, len(), string methods, lists, dictionaries, tuples, files (open), try / except, comprehensions, modules (import), and classes (class, __init__, self, methods) — plus this week's new material:

  • __str__ / __repr__ — special ("dunder") methods that decide how your object prints. __str__ is the friendly text for print(); __repr__ is the exact, developer-facing text used in the REPL and inside lists. Both return a string.
  • __eq__ — decide when two objects count as equal: def __eq__(self, other): returns True/False by comparing their data. Now == and in work on your type.
  • Class attributes — a value written straight in the class body (not on self) is shared by every object. Perfect for constants and for counting how many objects exist: Thing.count += 1 inside __init__.
  • Inheritance — class Dog(Animal): makes Dog a kind of Animal; it gets all of Animal's methods and can override any of them.
  • super().__init__(...) — inside a subclass's __init__, run the parent's setup first, then add the subclass's own attributes.
  • @property — turn a method into a computed attribute you read without parentheses: rect.area instead of rect.area().

Run each file with:

python exercise_1.py

exercise_1.py — A class that prints nicely

Define a class Book with a title and an author. Add a __str__(self) method that returns a sentence like "Python 101 by nino". Build two books and print each one.

Example run:

Python 101 by nino
Clean Code by giorgi

Hint: def __str__(self): then return f"{self.title} by {self.author}". Once __str__ exists, print(book) uses it automatically.

exercise_2.py — When are two things equal?

Define a class Coordinate with an x and a y. Add __eq__(self, other) so that two coordinates are equal when both parts match. Print the result of comparing an equal pair and an unequal pair.

Example run:

True
False

Hint: def __eq__(self, other): return self.x == other.x and self.y == other.y. Build a = Coordinate(3, 7), b = Coordinate(3, 7), c = Coordinate(9, 0), then print(a == b) and print(a == c).

exercise_3.py — Count how many you made

Give a class (say Robot) a class attribute count = 0 written in the class body. In __init__, do Robot.count += 1. Build three robots, then print how many were made using Robot.count.

Example run:

Made 3 robots.

Hint: count = 0 goes directly under class Robot: — not on self. Each new robot bumps the shared counter with Robot.count += 1.

exercise_4.py — A family of classes

Write a base class Animal with a method speak(self) that returns a generic sound. Write two subclasses (for example Dog and Cat) that override speak. Build one of each and print what each says.

Example run:

Rex says woof
Whiskers says meow

Hint: class Dog(Animal): puts the parent in the parentheses. Give Animal an __init__ that stores self.name, and let the subclasses reuse it — only speak needs to change.

exercise_5.py — Build on the parent

Write a base class Vehicle with __init__(self, brand). Write a subclass Car whose __init__ also takes doors, calls super().__init__(brand) to reuse the parent's setup, and then stores self.doors. Print a car's brand and doors.

Example run:

Toyota with 4 doors

Hint: class Car(Vehicle): then def __init__(self, brand, doors): super().__init__(brand); self.doors = doors. After that, self.brand exists (the parent set it) and so does self.doors.

exercise_6.py — A computed value

Write a class Rectangle with a width and a height. Add an @property called area that returns width * height. Build a rectangle and print its area — with no parentheses (rect.area, not rect.area()).

Example run:

Area: 12

Hint: put @property on the line directly above def area(self):. Then read it like an attribute: print(f"Area: {rect.area}").

exercise_7.py — Put it together

Write a base class Shape and two subclasses Square and Circle. Each subclass stores what it needs (a side, or a radius) and has its own @property area and a __str__. Put a few shapes in a list and print each one.

Example run:

Square with area 16
Circle with area 28.27

Hint: give Shape a __str__ that uses self.area, and let each subclass fill in its own area. A circle's area is math.pi * r * r (import math). Format to two decimals with f"{self.area:.2f}".

exercise_8.py — A library that remembers (bonus)

Combine classes, json, and error handling. Write a class Library whose __init__ tries to json.load a list of books from library.json (each book is a small dict like {"title": ..., "author": ...}); catch FileNotFoundError and start with an empty list if the file is missing. Add a method add(self, title, author) that appends a book dict and saves the whole list back with json.dump, and a method show(self) that prints every book as "title by author". Run the program two or three times, adding a book each time, and watch the shelf grow.

Example first run:

0 book(s) on the shelf.
Add a title: The Hobbit
Add an author: tolkien
Saved!

Example second run:

1 book(s) on the shelf.
- The Hobbit by tolkien
Add a title: Dune
Add an author: herbert
Saved!

Hint: in __init__, wrap the read in try: ... except FileNotFoundError: self.books = []. In add, do self.books.append({"title": title, "author": author}), then re-open library.json in "w" mode and json.dump(self.books, f).

Checklist before you submit

  • exercise_1.py … exercise_8.py written and run without errors using python exercise_N.py
  • Everything is pushed inside the workshop_9_homework/ folder of your repository
  • You re-sent your repository link to the instructor

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