Adding a value to a variable in Django templates -

this quick question, think may limitation in templates.

i'm adding value variable in template using |add:"num" feature, i'm trying use variable number doesn't seem work. should possible ?

e.g.

{{ pagecontrol.start_page|add:"-pagecontrol.news_per_page" }} 

thanks in advance all.

wayne

django not provide built-in template tag/filter subtraction. also, there no built-in filter converting positive integer negative. have write template tag that.

solution 1- use custom template filter:

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

then in template, can like:

{{ pagecontrol.start_page|subtract:pagecontrol.news_per_page }} 

solution 2- use django-mathfilters library

you can use django-mathfilters library subtract filter.

first, load mathfilters @ top of template. use sub filter in template below:

{% load mathfilters %}  {{ pagecontrol.start_page|sub:pagecontrol.news_per_page }} 

why code not working?

django source code of add built-in filter:

@register.filter(is_safe=false) def add(value, arg):     """adds arg value."""     try:         return int(value) + int(arg)     except (valueerror, typeerror):         try:             return value + arg         except exception:             return '' 

when {{ pagecontrol.start_page|add:"-pagecontrol.news_per_page" }} in template, '-pagecontrol.news_per_page' string passed arg above add built-in filter , not actual value, exception raised because of adding integer , string values. '' returned final value add filter nothing displayed.

note:

in case of complex variables name, can use with template tag. cache complex variable under simpler name.

below sample example of using with add filter. add values.

{% my_variable=pagecontrol.news_per_page %} 

this store value of pagecontrol.news_per_page in variable my_variable. in template, can use variable:

{{ pagecontrol.start_page|add:my_variable }} 

this add 2 variables.

{% my_variable=pagecontrol.news_per_page %}      {{ pagecontrol.start_page|add:my_variable }}  {% endwith %}