Capstone: OOP Library System

Lesson 10 · OKSTEM College · Associate of Science in Computer Science

Capstone: OOP Library System

You will design and implement a small library management system using every OOP concept from this course. This is an open-ended project — implement it in your local Python environment and test with pytest.

Requirements

Abstract base class LibraryItem(ABC) — abstract properties: title, item_id; abstract method: checkout_duration_days.
Concrete subclasses: Book, DVD, Magazine — each with appropriate attributes and checkout durations.
Encapsulation: Member class with @property for name (strip whitespace), email (validate @ present).
Class variable: Library.item_count tracks total items added across all Library instances.
Dunder methods: Book.__repr__, Book.__eq__ (same ISBN = same book), Book.__lt__ (sort by title).
Design pattern: Library acts as a Factory — Library.create_item(kind, **kwargs) returns the right subclass.
Tests: at least 8 pytest test functions covering checkout, equality, sorting, validation, and the factory.

Start small: get Book and Member working first, then add Library, then the factory and tests.

Starter Skeleton

from abc import ABC, abstractmethod from typing import Optional class LibraryItem(ABC): @property @abstractmethod def title(self) -> str: ... @property @abstractmethod def item_id(self) -> str: ... @abstractmethod def checkout_duration_days(self) -> int: ... class Book(LibraryItem): def __init__(self, title: str, author: str, isbn: str): self._title = title self.author = author self.isbn = isbn # TODO: implement abstract members + __repr__, __eq__, __lt__ class Member: def __init__(self, name: str, email: str): self.name = name self.email = email self.checked_out: list = [] # TODO: add @property with validation for name and email class Library: item_count = 0 # class variable def __init__(self): self.inventory: list = [] # TODO: add_item, checkout, return_item, create_item factory classmethod

Checklist

All three item subclasses instantiate correctly
Attempting to instantiate LibraryItem directly raises TypeError
Two Book objects with the same ISBN compare equal
A list of Book objects sorts alphabetically by title using sorted()
Member rejects an email without @
Library.create_item('book', ...) returns a Book instance
All 8+ pytest tests pass

Finished early? Add a Reservation system using the Observer pattern — Member subscribes to an item and is notified when it becomes available.

Knowledge Check

Which OOP concept prevents instantiating LibraryItem directly?

Encapsulation is about hiding state, not preventing instantiation.

Correct — unimplemented abstract methods cause TypeError on instantiation.

Name-mangling doesn't prevent instantiation.

Singleton limits to one instance; ABC prevents any instance.

Recap: class LibraryItem(ABC) with @abstractmethod stubs forces every concrete subclass to implement those methods.

Book.__eq__ based on ISBN means

Only books with the same ISBN are equal.

Correct — ISBN uniquely identifies a book; two copies have the same ISBN.

Sorting uses __lt__, not __eq__.

Defining __eq__ sets __hash__ to None; you'd also need __hash__ for set use.

Recap: define equality semantics deliberately. For a Book, same ISBN = same book (value equality), even if they're different Python objects.

sorted(books) works on a list of Book objects because

Without __lt__, sort raises TypeError.

Correct — sorted() uses __lt__ to order elements.

LibraryItem doesn't define __lt__ in the starter skeleton.

sorted() doesn't use __repr__; it uses comparison operators.

Recap: implement __lt__ (and ideally use @functools.total_ordering) to make your objects sortable with sorted() and .sort().

Library.create_item() is an example of

Observer is for event notification.

Correct — the factory decides which class to instantiate based on input.

Singleton controls instance count, not object type.

Factory is a design pattern; abstract method is a language feature.

Recap: Library.create_item('dvd', ...) returns a DVD — the caller doesn't import DVD directly. Easy to extend with new item types.

A property that validates email should raise ValueError when

Length wasn't listed as a requirement; the key check is the @ symbol.

Correct — minimum validation: an email must contain @.

Dots are valid in email addresses.

Lowercase is perfectly valid for email.

Recap: property setters are ideal validation points. Raise ValueError with a descriptive message so the caller knows exactly what was wrong.

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