Background

If you know me, then you know that I’m an avid tester. It could even be argued that I test too extensively as part of my day-to-day development, but that’s a post for another day.

On a recent project, a particular view started failing with the error:

AttributeError: 'ContextList' object has no attribute 'get'

I wasn’t happy with just changing the tests to work again, so I dug down into why they started failing.

TL;DR

To get a value from a Context object returned by the Django Test Client, then it’s better to use the [] operator than the get method.

For example:

# In a test, after doing
response = self.client.get(reverse('home'))

# ... then it's better to use [] to test the context
self.assertEqual(response.context['name'], 'Homer')

# ...than to use get
self.assertEqual(response.context.get('name'), 'Homer')

Reason

It turns out that the problem was to do with a developer on the project changing how the template for the view was generated. They changed a view that was using a single template, to a couple of templates using Django’s template inheritance and the extends template tag.

This then effects how Django’s test client returns the Context object for inspection.

To test this I prepared the following test:

from django.core.urlresolvers import reverse
from django.test import TestCase


class Tests(TestCase):

    def test_get(self):
        response = self.client.get(reverse('home'))
        self.assertEqual(response.context.get('name'), 'Homer')

    def test_operator(self):
        response = self.client.get(reverse('home'))
        self.assertEqual(response.context['name'], 'Homer')

The home view was just a simple template renderer:

from django.shortcuts import render


def home(request):
    return render(request, 'home.html', {'name': 'Homer', })

<p>Hello World</p>

./manage.py test
Creating test database for alias 'default'...
..
----------------------------------------------------------------------
Ran 2 tests in 0.027s

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

{% extends 'base.html' %}
<p>Hello World</p>

./manage.py test
Creating test database for alias 'default'...
E.
======================================================================
ERROR: test_get (mini.tests.Tests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/james/active/mini/mmm/mini/tests.py", line 9, in test_get
      self.assertEqual(response.context.get('name'), 'Homer')
AttributeError: 'ContextList' object has no attribute 'get'

----------------------------------------------------------------------
Ran 2 tests in 0.029s

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

Conclusion

It’s definitely more robust to be using list access [] on Context objects returned by the Django Test Client where possible when checking values passed through to templating.

Thanks for reading.