Django: Fail in templates with {% url '' %}

Previously, I covered using 1/0 to crash Python within minimal typing, useful to quickly answer questions like “does the code even get here?”. Recently, I wanted to do the same in Django templates, to trace if a given template was even being rendered, and under which code paths.

It’s a bit more challenging to deliberately crash Django’s template language, since the whole system is designed to be robust, silencing many kinds of errors. The language is also restricted, so {{ 1/0 }} doesn’t work, as variables cannot contain expressions.

Still, after playing around for a bit, I came up with following two options.

First, there’s a way to run 1/0, using divisibleby:

{{ 1|divisibleby:0 }}

On rendering, this raises a classic ZeroDivisionError:

ZeroDivisionError: integer modulo by zero

That said, divisibleby is a bit long and tricky to type.

Second, my preferred method uses the url tag:

{% url '' %}

The {% url %} looks up URLs by name, so providing it the empty string forces no URL to match, raising a NoReverseMatch error:

NoReverseMatch: Reverse for '' not found. '' is not a valid view function or pattern name.

I don’t think there is a shorter way to crash using only the built-in tags and filters, since most have names longer than {% url %}. If you want to crash often, you could write a custom tag, but I think the above is sufficient for most purposes.

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Related posts:

• Python: Fail in three characters with 1/0
• Django: launch pdb in templates with a custom {% breakpoint %} tag

Tags: django