Hunting the Shmoo

That’s not a rhetorical question. I’d like to know in which scenarios a mixin in python really is the best option. I can’t seem to think of any, but maybe I’m not thinking outside the box enough.

The basic idea of a mixin is to create a small re-usable class that can “plug-in” to other larger classes. From the wikipedia definition, a mixin is a way to compose classes together without using inheritance. The problem is unlike ruby, python mixins are a purely conceptual construct. Python mixins are inheritance (the only difference is that the class name usually contains ‘Mixin’). It is up to the developer to remember this, and to manually avoid all of the common pitfalls of multiple inheritance. This kind of defeats the whole purpose of the mixin in the first place. What’s more is that most people use python mixins improperly.

Consider the following example:

from book import Book
from index import IndexMixin

class TextBook(Book, IndexMixin):
    def __init__(self, name, contents, edition='1st'):
        Book.__init__(self, name, contents)
        self.edition = edition

t = TextBook('Learn Python the Hard Way')
t.search('python')
>>> 0 results found

Here we correctly use inheritance to denote Book as a base class, a textbook is a book. We use an IndexMixin because textbooks aren’t indices, they have an index. We then instantiate it and search the book for the string ‘python’. Wait a minute, how can a textbook about python not contain the word python? In python, the multiple inheritance chain is read right to left, this means that IndexMixin is technically the base class here. It just so happens that IndexMixin also has a function called search which overrides the one defined in Book.

It turns out there’s another way to compose classes together without using inheritance. Just use the class (or module) directly:

from book import Book
from index import Index

class TextBook(Book):
    def __init__(self, name, contents, edition='1st'):    
        Book.__init__(self, name, contents)
        self.edition = edition
        self.index = Index()

t = TextBook('Learn Python the Hard Way')
t.search('python')
>>> Many results found

t.index.search('python')
>>> 0 results found

If the relationship between objects A and B is “A is a B”, then B is a base class, not a mixin. If the relationship is “A has a B”, then why not use composition? What benefit does a mixin give you?