On mixins and the testing thereof

I make pretty heavy use of mixins when writing Django code. Mixins are great because they allow you to share common behaviour across disparate classes (and projects) with very little overhead. Class based views make heavy use of mixins, and I use them in a similar manner to compose views and forms that do a lot with seemingly little code.

On the other hand, mixins can be terrible because they can make it difficult to figure out exactly what a class does and what methods it provides. This is the source of many complaints about class based views. I've found that after I got over the initial learning curve, this style of composition has proved very powerful, and well worth the additional complexity.

One thing I had trouble with when adopting this mixin-oriented style was figuring out good ways to unit test the resulting code. It's easy enough to test mixins as independent objects, but usually what makes them powerful (and potentially hard to debug) is the interactions they have with the other classes into which they're mixed. For instance, it's usually important to ensure that your mixin's methods call the super class methods so that they add to rather than replace the behaviour of the super class.

Let's take an example mixin, which is intended to be used with a ModelForm, and which sets a property on the model before saving it.

class UpdatedByMixin(object):

    def save(self, *args, **kwargs):
        self.instance.updated_by = self.user

This looks great, but we've forgotten to call super. This mixin will correctly update the model instance, but the model will never be saved and our change will be lost. We could have avoided that bug by writing a test to ensure that the super method gets called. How do we do that?

Let's start by defining a form that we'll use just for testing this behaviour. It will mix together our UpdatedByMixin with a ModelForm.

class UpdatedByForm(UpdatedByMixin, ModelForm):

    class Meta:
        model = MyModel

    def __init__(self, user, *args, **kwargs):
        self.user = user
        super(UpdatedByForm, self).__init__(self, *args, **kwargs)

We also need a couple of test doubles to get the model form working. At minimum, a ModelForm needs an instance, and the form we defined requires a User to set the updated_by property.

class ModelStub(object):
    pass

class UserStub(object):
    pass

And finally we can write our test. The key here is to use the Mock library to patch the call to super, and then verify that the call was made.

from mock import patchclass

TestUpdatedByMixin(TestCase):

    @patch('django.forms.ModelForm.save')
    def test_calls_super_class_save_method(self, superclass_save):
        form = UpdatedByForm()
        form.user = UserStub()
        form.instance = ModelStub()

        form.save()

        self.assertTrue(superclass_save.called)

This test will fail, because our mixin is not calling its super class's save method.

$ python manage.py testCreating test database for alias 'default'...
F
======================================================================

FAIL: test_calls_super_class_save_method (exampleapp.tests.TestUpdatedByMixin)
----------------------------------------------------------------------
Traceback (most recent call last):

File "/home/jscn/work/code/temp/local/lib/python2.7/site-packages/mock.py", line 1201, in patched
return func(*args, **keywargs)

File "/home/jscn/work/code/temp/supertest/exampleapp/tests.py", line 27, in test_calls_super_class_save_method
self.assertTrue(superclass_save.called)
AssertionError: False is not true

----------------------------------------------------------------------
Ran 1 test in 0.002s

FAILED (failures=1)
Destroying test database for alias 'default'...

Now we can write the code to make it pass:

class UpdatedByMixin(object):

    def save(self, *args, **kwargs):
        self.instance.updated_by = self.user
        super(UpdatedByMixin, self).save(*args, **kwargs)

$ python manage.py testCreating test database for alias 'default'...
.

----------------------------------------------------------------------
Ran 1 test in 0.001s

OK
Destroying test database for alias 'default'...

Mixins are a powerful way to share behaviour across objects. It's vital to ensure that your mixins correctly extend the behaviour of the classes into which they're mixed. With the Mock library you can ensure that your mixin classes make the appropriate calls.