python - Django Model Mixins: inherit from models.Model or from object?

Question

This is a question about Python Mixins that might be useful in general. I'm just using Django models as that is the use-case I'm most familiar with.

Should a mixin inherit from the class it is designed to mix-in with or from 'object'?

Examples by code, what is more correct or better, or better depending on what you want to achieve?

This

class TaggingMixin(models.Model):
    tag = models.ForeignKey(Tag)

    class Meta:
        abstract = True

class MyModel(models.Model, TaggingMixin):
    title = models.CharField(max_length=100)

Or this:

class TaggingMixin(object):
    tag = models.ForeignKey(Tag)

    class Meta:
        abstract = True

class MyModel(models.Model, TaggingMixin):
    title = models.CharField(max_length=100)

I think inheriting from object is the right way. But I'm seeing examples of the first case all over the net...

EDIT: I've moved my follow up question to a separate question: Django Abstract Models vs simple Python mixins vs Python ABCs

Answer 1

Django does a lot of meta magic when it comes to its model classes, so unfortunately the usual approach to mixins as suggested in Daniel Roseman's answer -- where they inherit from object -- does not work well in the Django universe.

The correct way to structure your mixins, using the example provided, would be:

class TaggingMixin(models.Model):
    tag = models.ForeignKey(Tag)

    class Meta:
        abstract = True

class MyModel(TaggingMixin):
    title = models.CharField(max_length=100)

Important points here being:

• Mixins inherit from model.Model but are configured as an abstract class.
• Because mixins inherit from model.Model, your actual model should not inherit from it. If you do, this might trigger a consistent method resolution order exception.