library_management_system.py

"""
LIBRARY MANAGEMENT SYSTEM
--------------------------
A console-based Library Management System built using core Python concepts:
- OOP (Classes, Objects, Inheritance, Encapsulation, Polymorphism)
- File Handling (JSON read/write using context managers)
- Exception Handling (Custom Exceptions)
- Loops, Dictionaries, Functions

Author: Ume Rubab
"""

import json
import os

DATA_FILE = "library_data.json"


# ---------------------------------------------------------
# CUSTOM EXCEPTIONS
# ---------------------------------------------------------
class BookNotAvailableError(Exception):
    """Raised when a requested book has no available copies."""
    pass


class MemberLimitExceededError(Exception):
    """Raised when a member tries to borrow more books than allowed."""
    pass


class BookNotFoundError(Exception):
    """Raised when a book ID does not exist in the library."""
    pass


class MemberNotFoundError(Exception):
    """Raised when a member ID does not exist in the library."""
    pass


# ---------------------------------------------------------
# BOOK CLASS
# ---------------------------------------------------------
class Book:
    def __init__(self, book_id, title, author, total_copies):
        self.book_id = book_id
        self.title = title
        self.author = author
        self.total_copies = total_copies
        self.available_copies = total_copies

    def to_dict(self):
        return {
            "book_id": self.book_id,
            "title": self.title,
            "author": self.author,
            "total_copies": self.total_copies,
            "available_copies": self.available_copies
        }

    @staticmethod
    def from_dict(data):
        book = Book(data["book_id"], data["title"], data["author"], data["total_copies"])
        book.available_copies = data["available_copies"]
        return book

    def __str__(self):
        return f"[{self.book_id}] {self.title} by {self.author} | Available: {self.available_copies}/{self.total_copies}"
    

    # ---------------------------------------------------------
# MEMBER CLASS (Base Class - Encapsulation + Polymorphism)
# ---------------------------------------------------------
class Member:
    def __init__(self, member_id, name):
        self.member_id = member_id
        self.name = name
        self._borrowed_books = []   # protected attribute (encapsulation)

    def get_borrow_limit(self):
        """To be overridden by child classes (Polymorphism)."""
        return 2

    def get_borrowed_books(self):
        return self._borrowed_books

    def add_borrowed_book(self, book_id):
        self._borrowed_books.append(book_id)

    def remove_borrowed_book(self, book_id):
        self._borrowed_books.remove(book_id)

    def to_dict(self):
        return {
            "member_id": self.member_id,
            "name": self.name,
            "type": self.__class__.__name__,
            "borrowed_books": self._borrowed_books
        }

    def __str__(self):
        return f"[{self.member_id}] {self.name} ({self.__class__.__name__}) | Borrowed: {len(self._borrowed_books)}/{self.get_borrow_limit()}"


class Student(Member):
    def get_borrow_limit(self):
        return 2   # Students can borrow max 2 books


class Teacher(Member):
    def get_borrow_limit(self):
        return 5   # Teachers can borrow max 5 books
    

    # ---------------------------------------------------------
# LIBRARY CLASS (Manages everything)
# ---------------------------------------------------------
class Library:
    def __init__(self):
        self.books = {}      # book_id -> Book object
        self.members = {}    # member_id -> Member object
        self.load_data()

    # ---------------- FILE HANDLING ----------------
    def load_data(self):
        """Load books & members from JSON file if it exists."""
        if os.path.exists(DATA_FILE):
            try:
                with open(DATA_FILE, "r") as f:
                    data = json.load(f)
                    for b in data.get("books", []):
                        book = Book.from_dict(b)
                        self.books[book.book_id] = book
                    for m in data.get("members", []):
                        if m["type"] == "Teacher":
                            member = Teacher(m["member_id"], m["name"])
                        else:
                            member = Student(m["member_id"], m["name"])
                        member._borrowed_books = m["borrowed_books"]
                        self.members[member.member_id] = member
            except (json.JSONDecodeError, KeyError):
                print("⚠ Warning: Data file was corrupted. Starting fresh.")

    def save_data(self):
        """Save current books & members into JSON file."""
        data = {
            "books": [book.to_dict() for book in self.books.values()],
            "members": [member.to_dict() for member in self.members.values()]
        }
        try:
            with open(DATA_FILE, "w") as f:
                json.dump(data, f, indent=4)
        except IOError as e:
            print(f"Error saving data: {e}")

            # ---------------- BOOK OPERATIONS ----------------
    def add_book(self, book_id, title, author, copies):
        if book_id in self.books:
            print("A book with this ID already exists!")
            return
        self.books[book_id] = Book(book_id, title, author, copies)
        self.save_data()
        print(f"Book '{title}' added successfully.")

    def view_books(self):
        if not self.books:
            print("No books in the library yet.")
            return
        print("\n--- ALL BOOKS ---")
        for book in self.books.values():
            print(book)

    # ---------------- MEMBER OPERATIONS ----------------
    def add_member(self, member_id, name, member_type):
        if member_id in self.members:
            print("A member with this ID already exists!")
            return
        if member_type.lower() == "teacher":
            self.members[member_id] = Teacher(member_id, name)
        else:
            self.members[member_id] = Student(member_id, name)
        self.save_data()
        print(f"Member '{name}' added successfully as {member_type}.")

    def view_members(self):
        if not self.members:
            print("No members registered yet.")
            return
        print("\n--- ALL MEMBERS ---")
        for member in self.members.values():
            print(member)

            # ---------------- BORROW / RETURN (Exception Handling) ----------------
    def borrow_book(self, member_id, book_id):
        try:
            if member_id not in self.members:
                raise MemberNotFoundError(f"Member ID {member_id} not found.")
            if book_id not in self.books:
                raise BookNotFoundError(f"Book ID {book_id} not found.")

            member = self.members[member_id]
            book = self.books[book_id]

            if book.available_copies <= 0:
                raise BookNotAvailableError(f"'{book.title}' is currently not available.")

            if len(member.get_borrowed_books()) >= member.get_borrow_limit():
                raise MemberLimitExceededError(
                    f"{member.name} has reached the borrow limit ({member.get_borrow_limit()})."
                )

        except (MemberNotFoundError, BookNotFoundError, BookNotAvailableError, MemberLimitExceededError) as e:
            print(f"❌ Cannot borrow book: {e}")
        else:
            book.available_copies -= 1
            member.add_borrowed_book(book_id)
            self.save_data()
            print(f"✅ '{book.title}' borrowed successfully by {member.name}.")
        finally:
            print("Borrow operation finished.\n")

    def return_book(self, member_id, book_id):
        try:
            if member_id not in self.members:
                raise MemberNotFoundError(f"Member ID {member_id} not found.")
            if book_id not in self.books:
                raise BookNotFoundError(f"Book ID {book_id} not found.")

            member = self.members[member_id]
            book = self.books[book_id]

            if book_id not in member.get_borrowed_books():
                print(f"⚠ {member.name} hasn't borrowed this book.")
                return

        except (MemberNotFoundError, BookNotFoundError) as e:
            print(f"❌ Cannot return book: {e}")
        else:
            member.remove_borrowed_book(book_id)
            book.available_copies += 1
            self.save_data()
            print(f"✅ '{book.title}' returned successfully by {member.name}.")
        finally:
            print("Return operation finished.\n")


            # ---------------------------------------------------------
# MENU-DRIVEN MAIN PROGRAM
# ---------------------------------------------------------
def main():
    library = Library()

    while True:
        print("\n========== LIBRARY MANAGEMENT SYSTEM ==========")
        print("1. Add Book")
        print("2. View All Books")
        print("3. Add Member")
        print("4. View All Members")
        print("5. Borrow Book")
        print("6. Return Book")
        print("7. Exit")
        print("=================================================")

        choice = input("Enter your choice (1-7): ").strip()

        if choice == "1":
            book_id = input("Enter Book ID: ").strip()
            title = input("Enter Title: ").strip()
            author = input("Enter Author: ").strip()
            try:
                copies = int(input("Enter Number of Copies: ").strip())
                library.add_book(book_id, title, author, copies)
            except ValueError:
                print("❌ Copies must be a number.")

        elif choice == "2":
            library.view_books()

        elif choice == "3":
            member_id = input("Enter Member ID: ").strip()
            name = input("Enter Name: ").strip()
            member_type = input("Enter Type (Student/Teacher): ").strip()
            library.add_member(member_id, name, member_type)

        elif choice == "4":
            library.view_members()

        elif choice == "5":
            member_id = input("Enter Member ID: ").strip()
            book_id = input("Enter Book ID: ").strip()
            library.borrow_book(member_id, book_id)

        elif choice == "6":
            member_id = input("Enter Member ID: ").strip()
            book_id = input("Enter Book ID: ").strip()
            library.return_book(member_id, book_id)

        elif choice == "7":
            print("Thank you for using the Library Management System. Goodbye!")
            break

        else:
            print("❌ Invalid choice. Please enter a number between 1-7.")


if __name__ == "__main__":
    main()