How to set a variable in Django template

Posted by on Friday, February 18, 2011 · 22 Comments 

So, you need for some reason to set a variable inside a Django template? As you may have figured by now, Django does not implement such a feature. That is, because the logic of setting the variable must go in the view, not in the template. But there are some rare cases when you actually need this.

The way you can solve this is by using a custom Django template tag.

To do this, you create a file called ‘set_var.py’ in your templatetags forlder that contains the following code:

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from django import template
 
register = template.Library()
 
class SetVarNode(template.Node):
 
    def __init__(self, var_name, var_value):
        self.var_name = var_name
        self.var_value = var_value
 
    def render(self, context):
        try:
            value = template.Variable(self.var_value).resolve(context)
        except template.VariableDoesNotExist:
            value = ""
        context[self.var_name] = value
        return u""
 
def set_var(parser, token):
    """
        {% set <var_name>  = <var_value> %}
    """
    parts = token.split_contents()
    if len(parts) < 4:
        raise template.TemplateSyntaxError("'set' tag must be of the form:  {% set <var_name>  = <var_value> %}")
    return SetVarNode(parts[1], parts[3])
 
register.tag('set', set_var)

Then to use this in your template, you just do the following:

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{% load set_var %}
 
{% set a = 3 %}
{% set b = some_context_variable %}
{% set c = "some string" %}
 
Value of a is {{a}}
Value of b is {{b}}
Value of c is {{c}}

As you can see, you can put in your variables anything that you could normally get inside a Django template: a number, another context variable, a string, etc.

Hope it helped ;)

Filed under Coding · Tagged with

Comments

22 Responses to “How to set a variable in Django template”
  1. Maidot says:
    May 31, 2011 at 10:11

    Thanks you~ it’s a useful way. :)

    Reply
  2. François Schiettecatte says:
    July 23, 2011 at 01:28

    Hi

    I have been using this today and it does not work in this case:

    {% set var = 0 %}

    {% for history in page.historyList %}
    {% set var = 1 %}
    {% endfor %}

    {{ var }} # this render 0

    I am assuming this is because the ‘context’ passed to ‘render’ is different in both cases, any ideas on how to get around that?

    Thanks

    Reply
    • Nicolae Surdu says:
      July 23, 2011 at 15:53

      I’ll have to check this on Monday, but I believe is due to the case that the tag will set the varable in the current context, and for’s have their own context. Again, I’ll have to check this in Monday.

      Reply
      • François Schiettecatte says:
        July 26, 2011 at 16:26

        I expect that is what is going on, I had to work around it. Thanks for the code nonetheless, I used it elsewhere and was able to refactor out hacks I had put in.

        Reply
  3. Johanna says:
    September 27, 2011 at 11:39

    Thanks for the tip ! Saved me a lot of frustration :)

    Reply
  4. Alex Boschmans says:
    November 24, 2011 at 19:21

    I needed this to make make a nest of unordered lists that could be used by javascript to turn into a ‘tree’ – this helped nicely to determine the level of the tree Thanks !!

    Reply
  5. matias says:
    November 30, 2011 at 22:50

    hi, i tried setting a var inside a if template tag and dosnt work:

    {% if imagen.imagen.width >= 400 %}
    {% set c = “some string” %}
    {% endif %}

    {{c}}

    this dosnt do anything.

    Any ideas about this?

    Reply
  6. Kevin Godden says:
    February 24, 2012 at 18:29

    Thanks for the post, very useful. It’s a pity that django doesn’t have support for this out of the box!

    Reply
  7. dhiru says:
    July 17, 2012 at 11:58

    how to increment or add with another number SET variable

    Reply
    • Nicolae Surdu says:
      July 17, 2012 at 14:32

      The tag code will have to be slightly adjusted so that parts[3] is evaluated before is used as a value. If I’ll have time I’ll implement your idea, shouldn’t be hard …

      Reply
  8. rkjdid says:
    October 30, 2012 at 17:13

    quick dirty fix for context problems, only dealing with the upper-most context.
    This can be done by changing any context call by context.dicts[0] :

    example :
    context[self.var_name] = value
    becomes :
    context.dicts[0][self.var_name] = value

    Reply
  9. plex says:
    November 15, 2012 at 07:53

    {% for topic in my_topics %}
    {% set already_follow = true %}
    {% include “topic_item.html”%}
    {% endfor %}

    already_follow will not work in topic_item.html
    What should I do?

    Reply
  10. Paul says:
    November 30, 2012 at 18:11

    Isn’t the {% with %} tag enough?

    https://docs.djangoproject.com/en/1.4/ref/templates/builtins/

    Reply
  11. foo says:
    February 12, 2013 at 01:10

    Trying this with google app engine, I’ve tried everything, still getting: TemplateSyntaxError: ‘set_var’ is not a valid tag library: Template library set_var not found, tried google.appengine._internal.django.templatetags.set_var

    Reply
  12. bar says:
    March 29, 2013 at 08:15

    Here’s a snippet of my code,

    {% if forloop.last %}
    {% set flag = 1 %} {%else%} {% set flag = 0%}
    {%endif%}
    .
    .
    {% if flag == 1 %}
    do this
    {% endif %}

    It does not check the condition for if. How do I make it work? Sorry if the question is a bit too naive, I’m new to this.

    Reply
  13. nitin says:
    April 9, 2013 at 13:56

    {% set a = 3 %}
    {% set b = 3 %}
    {% if a==b %}
    print
    {% else %}
    none
    {% endif %}

    ### why if loop is not working…???

    Reply

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