Some bugs are hard to even notice. For example, what if you forgot to add a title to one of your web pages? Oops! You're probably left with a generic title of only your site name, or worse, no title at all. And it's very tricky to spot this. Most people testing a page won't even look at the title, they will go straight to the content.
We could write a script to track down all the pages without a title, but this is probably a time-consuming task, and you'll need to remember to run this, or put it in CI and slow your build down. But an ounce of prevention is worth a pound of cure. Let's make sure we can't forget to add a title for our pages. We can do this by putting a block on our base template that must be overridden in extending templates.
Let's assume we have a simple Django base template like this:
<!doctype html>
<html lang="tok">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>{% block title %}{% endblock %} - kulupu mi</title>
</head>
<body>
{% block content %}{% endblock %}
</body>
</html>
And our home page template extending it:
{% extends "base.html" %}
{% block content %}
<h1>kama pona</h1>
<p>toki! mi jan Tami!</p>
{% endblock %}
This will render the title as " - kulupu mi" on the home page. Oopsie.
Okay, let's actually fix this. All we need to do is add a template tag that raises an exception, and stick it in the right place in the base template.
from django import template
register = template.Library()
class BlockNotOverriddenError(NotImplementedError):
pass
@register.simple_tag
def ensure_overridden():
raise BlockNotOverriddenError
I choose to create a new exception class, BlockNotOverriddenError, to make it clear what the error is, and make it easier to test, based on a blog post on handling exceptions.
Then in our template, a single change on the title line:
<!doctype html>
<html lang="tok">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>{% block title %}{% ensure_overridden %}{% endblock %} - kulupu mi</title>
</head>
<body>
{% block content %}{% endblock %}
</body>
</html>
Now if we try to extend this without overriding the title block, we'll get an exception when we try to load the page, and our tests will fail. You did write a test for this page, right?
In your next (or current) Django project, don't run into this class of bug anymore. Throw in this single line template tag and improve your SEO.