Still confused with @classmethod

Question

I read all the questions here, but it's still unclear for me.
is @classmethod more like an annotation, or it actually functioning?
My best guess is, we can only use cls when we are using @classmethod, and when we use it, it works as if we call it outside class. so, therefore, just as if we did Bookcase(book_list). 
at the note,
python
def create_bookcase(self, book_list):
    for author, title in book_list:
        self.append(Book(author, title))
    return self

why are they attending to self, not to self.books?
Bookcase class never inherited list, but how can it use .append()?

Chris Freeman · Accepted Answer

Good question! You asked is @classmethod more like an annotation, or it actually functioning? 
The decorator (https://docs.python.org/3/glossary.html#term-decorator) @classmethod is more than an annotation, it marks the method as belonging to the class and makes the method assessable from the class. A classmethod does not require a class instance to run the method. 
In the same way the self of a "regular" instance method is assigned to point at the current class instance (done at instance creation), in a class method the first parameter cls is assigned to point to the class itself. This makes it straightforward to reference the class from within the class method.
My best guess is, we can only use cls when we are using @classmethod, and when we use it, it works as if we call it outside class.
Correct. The class method would be referenced using ClassName.method_name().  Notice there are no parens after the ClassName. If there were parens, that would cause an instance of the class to be created. Not trying to make things more confusing, a class instance my also access class methods from the instance though it is not the usual usage model for class methods.
*So, therefore, just as if we did Bookcase(book_list). * 
While it may be possible to have a class method that behaves the same as the __init__ method, in this situation, the Bookcase.__init__ method is looking for a list of Book instances, and the Bookcase.create_bookcase() classmethod is looking for a list of author, title pairs that it will use to create Book instances, and then append those to the books attribute.
why are they attending to self, not to self.books? Bookcase class never inherited list, but how can it use .append()?
You are correct! Bookcase itself does not have an append() method define. As is, the code in the notes would not work. Though it could with the additional following method:
python
    def append(self, item):
        self.books.append(item)

Post back if you wish more help. Good luck!!

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Juneau Lim

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Still confused with @classmethod

1 Answer

Chris Freeman

Chris Freeman

Juneau Lim

Juneau Lim