models.py file;

class Library(object):
    def __init__(self, name, budget):
        self.name = name
        self.remaining_balance = budget

    def funds_available(self, cost):
        if self.remaining_balance - cost > 0:
            return True
        else:
            print 'Insufficient funds'
            return False

    def open_new_branch(self, location, budget):
        if self.funds_available(budget):
            self.remaining_balance -= budget
            return Branch(location, self.name, budget)


class Branch(Library):
    def __init__(self, location, *args, **kwargs):
        self.location = location
        self.books = []
        super(Branch, self).__init__(*args, **kwargs)

    def funds_available(self, cost):
        return super(Branch, self).funds_available(cost)

    def purchase_book(self, title, isbn, genre, price):
        if self.funds_available(price):
            self.remaining_balance -= price
            book = LibraryBook(1, title, isbn, genre)
            self.books.append(book)

    def loan_book(self, isbn, patron):
        for book in self.books:
            if book.isbn == isbn:
                print '"%s" found in catalog' % book.title
                if book.status == 1:
                    book.status = 0
                    book.patron = patron
                    print '"%s" loaned to %s' % (book.title, patron)
                else:
                    print 'book is currently unavailable'

    def place_hold(self, isbn, patron):
        for book in self.books:
            if book.isbn == isbn:
                print '"%s" found in catalog' % book.title
                if book.status == 1:
                    book.status = 2
                    book.patron = patron
                    print '"%s" placed on hold for %s' % (book.title, patron)
                else:
                    print 'book is currently unavailable'

    def return_book(self, isbn):
        for book in self.books:
            if book.isbn == isbn:
                print '"%s" found in catalog' % book.title
                book.status = 1
                book.patron = None
                print '"%s" returned' % book.title

    def list_books(self):
        print 'Status       | Title'
        print '-'*40
        for book in self.books:
            print '%12s | %s' % (book.get_status(), book.title)


class Book(object):
    def __init__(self, title, isbn, genre):
        self.title = title
        self.isbn = isbn
        self.genre = genre


class LibraryBook(Book):
    def __init__(self, status, *args, **kwargs):
        self.STATUSES = {
            0: 'unavailable',
            1: 'on shelf',
            2: 'hold'
        }
        self.status = status
        self.patron = None
        super(LibraryBook, self).__init__(*args, **kwargs)

    def get_status(self):
        return self.STATUSES[self.status]

sample.py file;

from models import Library, Branch, LibraryBook

main_library = Library('Chicago Library', 100000)
edgewater_branch = main_library.open_new_branch('Edgewater', 25000)

edgewater_branch.purchase_book('History of Rome', '012345-1234', 'History', 150)
edgewater_branch.purchase_book('Scarlet Letter', '2222-1234', 'Fictional Literature', 1)
edgewater_branch.purchase_book('Starship Troopers', '3333-1234', 'Science Fiction', 15)
edgewater_branch.purchase_book('Weird Illinois', '0444-1234', 'Special Interest', 45)
edgewater_branch.purchase_book('The Holy Bible', '0555-1234', 'Religious Texts', 12)
edgewater_branch.purchase_book('The Holy Quran Translation', '0678-1234', 'Religious Texts', 12)
edgewater_branch.purchase_book('GSU Student Catalog', '0901-1234', 'Education Resources', 0)

edgewater_branch.loan_book('3333-1234', 'Dan')
edgewater_branch.loan_book('0444-1234', 'Dan')
edgewater_branch.loan_book('2222-1234', 'Dan')

edgewater_branch.place_hold('0901-1234', 'Dan')

edgewater_branch.list_books()
print '\n\n'

edgewater_branch.return_book('2222-1234')

edgewater_branch.list_books()