How to override getitem when it's a class method?

Question

class Custom(type):
    @classmethod
    def __getitem__(cls, item):
        raise NotImplementedError("")

    @classmethod
    def test(cls):
        print("class custom : test")

class Book(metaclass=Custom):
    Note = 0
    Pad = 1

    Name = { Note : "Note", Pad : "Pad"}

    @classmethod
    def __getitem__(cls, item):
        return Book.Name[item]

    @classmethod
    def test(cls):
        print("class book: test")

My intention is to have

Book[Book.Note] returns "Note"

It seems __getitem__() is not overrideable, unlike test(). How do I make it work ?

score 7 · Accepted Answer · 2015-05-31 04:10:04Z

7

You're using a metaclass here. This isn't strictly inheritance: you've defined the class of the class Book as Custom, whereas it used to be type. Because magic methods like __getitem__ are looked up directly on the class, and not on the instance, indexing Book[whatever] will actually call the __getitem__ method of the class of Book, which happens to be Custom.

My intention is to have
Book[Book.Note] 
returns "Note"

In that case, you should make the class of Book implement __getitem__ such that it returns "Note". Since the class of Book is Custom, this is where the change needs to be made:

class Custom(type):
    def __getitem__(cls, item):
        return cls.Name[item]
    ...

class Book(metaclass=Custom):
    ... # as is, although you don't need the @classmethod __getitem__

Book[Book.Note] # "Note"
Book[1]         # "Pad"

edited May 31, 2015 at 4:10

answered May 31, 2015 at 4:03

user1726343

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3 Comments

user1726343 Over a year ago

@user1502776 That's odd. I'm using Python 3 myself and it works. Let me see if I can put together a demo.

user1502776 Over a year ago

Your explanation demonstrates profound understanding of Python workings. Could you please recommend a book to read on ..?

user1726343 Over a year ago

@user1502776 I wouldn't say I have a rock solid understanding of Python, even after a long time using it, but if I had to suggest anything I'd suggest reading the docs; they're pretty good.

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