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Python Object-Oriented Python Advanced Objects Constructicons

Why the use of books=None?

I'm still working through this video to understand classmethods properly, so I might be missing something that seems obvious, but this use of books=None has stumped me, can anyone explain?

class Book:
    def __init__(self, title, author):
        self.title = title
        self.author = author

    def __str__(self):
        return '{} by {}'.format(self.title, self.author)

class Bookcase:
    def __init__(self, books=None):
        self.books = books

    @classmethod
    def create_bookcase(cls, book_list):
        books = []
        for title, author in book_list:
            books.append(Book(title, author))
            return cls(books)

bc = Bookcase.create_bookcase([('Moby-Dick', 'Herman Melville'), ('Jungle Book', 'Rudyard Kipling')])

In the video it says that "you shouldn't use a mutable object as the default argument to an argument", but I'm not exactly sure what that means .

Also, if anyone has the time, could they go through a step-by-step as to what's actually happening when the command is called? I'm struggling to follow what's being passed where!

2 Answers

Jonathan Grieve
MOD
Jonathan Grieve
Treehouse Moderator 90,689 Points

Mutable means an object that has values that can change. Something is immutable, im programming terms that means Objects that cannot be changed.

In OOP Python books=None is simply passing in a default value as a parameter to the init method of the BookCase class. You're assigning a default value to the books parameter on creation of an instance,

So what your code is doing is creating an initial instance with default values, and then creating a new instance that passes in values to your create_bookcase() method.

Cheers pal! I get that it's mutable, I guess I just don't really know why you shouldn't pass a mutable object as an argument. Up to now we've been passing mutable objects to methods, like lists or dicts. Is there a reason that it isn't just:

class Bookcase:
    def __init__(self):
        self.books = None

or:

class Bookcase:
    def __init__(self, books):
        self.books = books

I mean, we pass a list in straight away don't we?

Begana Choi
PLUS
Begana Choi
Courses Plus Student 13,123 Points

I also wonder about this ! :-/ looking forward clear answer! thank you!