Python: append() function not working when I print the list?

Question

I have a class object and tried to add that object to a class list but it doesn't work for some reason. What am I doing wrong here? I am new to Python. I know this kind of question would not fit in this community but I do need help from professional people like you guys. It would be appreciated if someone can help solve this! Thanks.

Here's my code snippet below:

class Catalogue:

    def __init__(self, catalogue_list):

        self._catalogue_list = catalogue_list

    def add_item(self):

        item = LibraryItemGenerator.create_item()
        self._catalogue_list.append(item)              # append not working?...
        print(item)

    def display_available_items(self):

        print("Item List")
        print("--------------", end="\n\n")
        for library_item in self._catalogue_list:
            print(library_item)


class LibraryItemGenerator:

    @staticmethod
    def create_item():

        print("What kind of items would you like to add to the library catalogue?")
        print("1. Book")
        print("2. DVD")
        print("3. Journal")

        option = int(input("Select type of item: "))

        title = input("Enter title: ")
        call_num = input("Enter call number: ")
        author = input("Enter author name: ")
        num_copies = input("Enter the number of copies: ")

        if option == 1:
            return Book(call_num, title, num_copies, author)

        if option == 2:
            release_date = input("Enter release date: ")
            region_code = input("Enter region code: ")
            return Dvd(call_num, title, num_copies, author, release_date, region_code)

        if option == 3:
            names = input("Enter name: ")
            issue_number = input("Enter issue number: ")
            publisher = input("Enter publisher: ")
            return Journal(call_num, title, num_copies, author, names, issue_number, publisher)

def main():

    book = Book("263.25A", "Burgers and Yam Fries", 3, "Dr. Hou")
    journal = Journal("267.21B", "I am hungry!", 5, "Dr. Hou", "THE NEWYORK TIMES", 21, "James Poul")
    dvd = Dvd("193.25C", "Overwatch is fun", 2, "Dr. Hou", "2020-06-23", "CA")
    cat = Catalogue([book, journal, dvd])

    cat.add_item()
    cat.display_available_items()   #doesn't show the newly created objects.. append() didn't work?


if __name__ == '__main__':
    main()

Answer 1

suggest using pdb debugger. just insert pdb.set_trace() between item = LibraryItemGenerator.create_item() and self._catalogue_list.append(item). Use commands to navigate what actually happen before and after calling append().

in pdb mode, you can do print(ietm, self._catalogue_list) and try self._catalogue_list.append(item). Then after calling self._catalogue_list.append(item), print(self._catalogue_list) again. compare it with your previous print [<Labs.Lab3.book.Book object at 0x10840bfd0>, <Labs.Lab3.journal.Journal object at 0x10840bf70>, <Labs.Lab3.dvd.Dvd object at 0x1084072e0>]

also i would suggest if..elif..elif instead of if..if..if. Former will raise error where you can catch if option not in [1, 2, 3]

Answer 2

Here is a fully functional code with all of the classes and methods fixed:

class Catalogue:

    def __init__(self, catalogue_list):
        self._catalogue_list = catalogue_list

    def add_item(self, item):
        self._catalogue_list.append(item)

    def display_available_items(self):

        print("Item List")
        print("--------------", end="\n\n")
        for library_item in self._catalogue_list:
            print(library_item)

class LibraryItemGenerator:
    def __init__(self, option):
        self.option = option
        self.title = input("Enter title: ")
        self.call_num = input("Enter call number: ")
        self.author = input("Enter author name: ")
        self.num_copies = input("Enter the number of copies: ")

        if option == 1:
            pass

        elif option == 2:
            self.release_date = input("Enter release date: ")
            self.region_code = input("Enter region code: ")

        elif option == 3:
            self.names = input("Enter name: ")
            self.issue_number = input("Enter issue number: ")
            self.publisher = input("Enter publisher: ")

    def __str__(self) -> str:
        #Provides a readable description of the object
        description = f'Title: {self.title}, ' \
                      f'Call No: {self.call_num}, ' \
                      f'Author: {self.author}, ' \
                      f'Number of Copies: {self.num_copies}'

        if self.option == 1:
            return "BOOK: " + description

        elif self.option == 2:
            description += f', Release date: {self.release_date}, ' \
                           f'Region code: {self.region_code} '
            return "DVD: " + description

        elif self.option == 3:
            description += f', Names: {self.names}, ' \
                           f'Issue number: {self.issue_number}, ' \
                           f'Publisher: {self.publisher}'
            return "JOURNAL: " + description

def main():
    cat = Catalogue([])
    while True:
        print("What kind of items would you like to add to the library catalogue?")
        print("1. Book")
        print("2. DVD")
        print("3. Journal")
        option = int(input("Select item type (or -1 to finish): "))
        if option == -1:
            break
        item = LibraryItemGenerator(option)
        cat.add_item(item)

    cat.display_available_items()


if __name__ == '__main__':
    main()

After querying the user, this is a sample output:

BOOK: Title: Odyssey, Call No: 23, Author: Homer, Number of Copies: 2
DVD: Title: Total Recall, Call No: 00234, Author: Picture Company, Number of Copies: 2, Release date: 1980, Region code: 908