Variable subtraction in django templates

It is able to write {{ myval.add:5 }}, {{ myval|add:value }} and even {{ myval|add:-5 }}.

However, I can't find out what I should type to add value * -1 like {{ myval|add:-value }}. This doesn't work, sadly.

You need to use double quotes:

{{ myval|add:"-5" }}

This subtracts five from myval.

The built-in Django template tags/filters aren't all-encompassing, but it's super easy to write your own custom template tags: https://docs.djangoproject.com/en/dev/howto/custom-template-tags/

You could make your own subtract template tag pretty easily:

@register.filter
def subtract(value, arg):
    return value - arg

Use django-mathfilters from PyPI: https://pypi.python.org/pypi/django-mathfilters

To install :

$ pip install django-mathfilters

Then add mathfilters in your INSTALLED_APPS.

In template:

{% load mathfilters %}

<ul>
    <li>8 + 3 = {{ 8|add:3 }}</li>

    <li>13 - 17 = {{ 13|sub:17 }}</li>

    {% with answer=42 %}
    <li>42 * 0.5 = {{ answer|mul:0.5 }}</li>
    {% endwith %}

    {% with numerator=12 denominator=3 %}
    <li>12 / 3 = {{ numerator|div:denominator }}</li>
    {% endwith %}

    <li>|-13| = {{ -13|abs }}</li>
</ul>