book.py

class Book:
    """A book of a library"""
    def __init__(self, title, author, borrowed=False):
        self.title = title
        self.author = author
        self.is_borrowed = borrowed

    def __str__(self):
        return f"Title: {self.title}, Author: {self.author}, Borrowed: {'Yes' if self.is_borrowed else 'No'}"

    def __repr__(self):
        return f"'{self.title}'"

class Library:
    """Library as list of books"""
    def __int__(self):
        self.books = []

    def add_book(self, book):
        self.books.append(book)

    def search_by_title(self, title):
        found_books = []

        for book in self.books:
            if book.title == title:
                 found_books.append(book)

        return found_books

    def search_by_author(self, author):
        found_author = []

        for book in self.books:
            if book.author == author:
                found_author.append(book) 

        return found_author

    def list_available_books(self):
        available_books = []

        for book in self.books:
            if not book.is_borrowed:
                available_books.append(book)

        return available_books

    def borrow_book(self, title):
        for book in self.books:
            if book.title == title:
                if not book.is_borrowed:
                    return book
                else:
                    print(f"Error: This book has been borrowed")
                    return None

        print(f"Error: No book with this title exists")
        # return None

    def return_book(self, book):
        if book.is_borrowed:
            book.is_borrowed = False
        else:
            print("Error: This book was not borrowed.")


    def remove_book(self, title):
        for book in self.books:
            if book.title == title:
                self.books.remove(book)

    # def sort_books_by_title(self):
    #     self.books.sort(key = self.books[] book: book.title)

this should be found_books, beacuse you are searching for books

this still needs to be implemented