How to set a value of a variable inside a template code?

Question

Say I have a template

<html>
<div>Hello {{name}}!</div>
</html>

While testing it, it would be useful to define the value of the variable without touching the python code that invokes this template. So I'm looking for something like this

{% set name="World" %}     
<html>
<div>Hello {{name}}!</div>
</html>

Does something like this exists in Django?

John · Accepted Answer · 2019-06-10T21:14:32.313

453

You can use the with template tag.

{% with name="World" %}     
<html>
<div>Hello {{name}}!</div>
</html>
{% endwith %}

edited Jun 10 '19 at 21:14

answered Jul 01 '09 at 17:38

John

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62

but can you change the variable's value in the with? – David 天宇 Wong Feb 16 '14 at 10:31
4

It seems you cannot declare a container (I've tried list and tuple) in a with clause – Vladislav Ivanishin Mar 18 '15 at 01:58
1

If you need to declare a list, use make_list. https://docs.djangoproject.com/en/1.9/ref/templates/builtins/#make-list – MrValdez Jul 01 '16 at 07:55
4

Jinja says it's {% set myvar=value %} why it doesn't work in django? – holms Mar 10 '18 at 01:08
14

@holms Because Django doesn't use Jinja :-) https://docs.djangoproject.com/en/1.7/topics/templates/ – elimisteve May 12 '18 at 00:04
@MrValdez update correct link: [Django-docs make_list filter](https://docs.djangoproject.com/en/stable/ref/templates/builtins/#make-list) for further reference. – nerdoc Feb 28 '23 at 14:34

score 87 · Answer 2 · edited Oct 09 '19 at 10:23

87

Create a template tag:

The app should contain a templatetags directory, at the same level as models.py, views.py, etc. If this doesn’t already exist, create it - don’t forget the __init__.py file to ensure the directory is treated as a Python package.

Create a file named `define_action.py` inside of the templatetags directory with the following code:

from django import template
register = template.Library()

@register.simple_tag
def define(val=None):
  return val

Note: Development server won’t automatically restart. After adding the templatetags module, you will need to restart your server before you can use the tags or filters in templates.

Then in your template you can assign values to the context like this:

{% load define_action %}
{% if item %}

   {% define "Edit" as action %}

{% else %}

   {% define "Create" as action %}

{% endif %}


Would you like to {{action}} this item?

edited Oct 09 '19 at 10:23

John R Perry

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answered Jun 10 '16 at 19:19

Mods Vs Rockers

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in my case after loop this returns old value :( – holms Mar 13 '18 at 09:46
8

In the latest version it appears that you can use simple_tag instead of assignment_tag (and it worked for me). – Katharine Osborne Mar 13 '18 at 19:44
Issue I got with this solution is that it appears, that you cannot override values. – Jakub Jabłoński Mar 13 '20 at 15:57
1

if you want to use this technique to set a list instead of just a value, check this: https://stackoverflow.com/a/34407158/2193235 – msb Jun 13 '20 at 01:45
2

if you are setting the variable as an integer and you want to increment it (for example), you need to use `add`: `{% define counter|add:1 as counter %}`. Similarly for other operations. – msb Jun 13 '20 at 01:47
How can define integer and increase of decrease? – parmer_110 Mar 30 '23 at 06:19

score 38 · Answer 3 · edited Nov 26 '14 at 14:21

An alternative way that doesn't require that you put everything in the "with" block is to create a custom tag that adds a new variable to the context. As in:

class SetVarNode(template.Node):
    def __init__(self, new_val, var_name):
        self.new_val = new_val
        self.var_name = var_name
    def render(self, context):
        context[self.var_name] = self.new_val
        return ''

import re
@register.tag
def setvar(parser,token):
    # This version uses a regular expression to parse tag contents.
    try:
        # Splitting by None == splitting by spaces.
        tag_name, arg = token.contents.split(None, 1)
    except ValueError:
        raise template.TemplateSyntaxError, "%r tag requires arguments" % token.contents.split()[0]
    m = re.search(r'(.*?) as (\w+)', arg)
    if not m:
        raise template.TemplateSyntaxError, "%r tag had invalid arguments" % tag_name
    new_val, var_name = m.groups()
    if not (new_val[0] == new_val[-1] and new_val[0] in ('"', "'")):
        raise template.TemplateSyntaxError, "%r tag's argument should be in quotes" % tag_name
    return SetVarNode(new_val[1:-1], var_name)

This will allow you to write something like this in your template:

{% setvar "a string" as new_template_var %}

Note that most of this was taken from here

How about assigning variables to other variables present in the context? And on a different note: allowing templates to arbitrarily assign context variables without checking if they exist already may have security implications. A more sensible approach in my opinion would be to check the context for the variable before attempting to assign it: — , Aug 07 '15 at 11:49
if context.get(self.var_name): raise SuspiciousOperation("Attempt to assign variable from template already present in context") — , Aug 07 '15 at 11:50

score 36 · Answer 4 · edited Mar 26 '21 at 07:53

36

There are tricks like the one described by John; however, Django's template language by design does not support setting a variable (see the "Philosophy" box in Django documentation for templates).
Because of this, the recommended way to change any variable is via touching the Python code.

edited Mar 26 '21 at 07:53

djvg

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answered Jul 01 '09 at 18:12

rob

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Thanks for the pointer. From a perspective of a designer is it sometimes easier to quickly set a variable to test various states of a page while designing it. Not suggesting this practice to be used in a running code. – Alexis Jul 01 '09 at 22:21
3

the "with" tag is accepted in django1.0. So looks like they are finally amending their philosophy :). – Evgeny Dec 20 '09 at 19:35
4

As a matter of facts, the "with" tag is just for aliases. This may have a huge impact on performance (and on readability as well!) but it is not really setting a variable in traditional programming terms. – rob Dec 20 '09 at 23:48

score 16 · Answer 5 · edited Apr 22 '21 at 02:31

The best solution for this is to write a custom assignment_tag. This solution is more clean than using a with tag because it achieves a very clear separation between logic and styling.

Start by creating a template tag file (eg. appname/templatetags/hello_world.py):

from django import template

register = template.Library()

@register.simple_tag
def get_addressee():
    return "World"

Now you may use the get_addressee template tag in your templates:

{% load hello_world %}

{% get_addressee as addressee %}

<html>
    <body>
        <h1>hello {{addressee}}</h1>
    </body>
</html>

For folks using the newer Django versions, its called simple_tag now! Save the time to figure out why "register.." is not recognized in your code... — kaya, Sep 01 '18 at 09:25

score 14 · Answer 6 · answered Dec 06 '16 at 02:12

14

Perhaps the default template filter wasn't an option back in 2009...

<html>
<div>Hello {{name|default:"World"}}!</div>
</html>

answered Dec 06 '16 at 02:12

John Mee

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I must say that this is what I was looking! It can be also be used with **with**: `{% with state=form.state.value|default:other_context_variable %}` instead of `other_context_variable` we can also use any `'string_value'` as well – Saurav Kumar Feb 07 '18 at 09:48
But it will print it, and I need to save it for later use – holms Mar 10 '18 at 01:18

score 3 · Answer 7 · answered Jul 19 '14 at 07:02

This is not a good idea in general. Do all the logic in python and pass the data to template for displaying. Template should be as simple as possible to ensure those working on the design can focus on design rather than worry about the logic.

To give an example, if you need some derived information within a template, it is better to get it into a variable in the python code and then pass it along to the template.

score 3 · Answer 8 · answered Jul 15 '16 at 08:43

In your template you can do like this:

{% jump_link as name %}
{% for obj in name %}
    <div>{{obj.helo}} - {{obj.how}}</div>
{% endfor %}

In your template-tags you can add a tag like this:

@register.assignment_tag
def jump_link():
    listArr = []
    for i in range(5):
        listArr.append({"helo" : i,"how" : i})
    return listArr

score 3 · Answer 9 · answered Jul 31 '17 at 06:29

3

Use the with statement.

{% with total=business.employees.count %}
    {{ total }} employee{{ total|pluralize }}
{% endwith %}

I can't imply the code in first paragraph in this answer. Maybe the template language had deprecated the old format.

answered Jul 31 '17 at 06:29

ramwin

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How to set a value of a variable inside a template code?

9 Answers9

Create a template tag:

Create a file named `define_action.py` inside of the templatetags directory with the following code:

Then in your template you can assign values to the context like this:

Linked

How to set a value of a variable inside a template code?

9 Answers9

Create a template tag:

Create a file named define_action.py inside of the templatetags directory with the following code:

Then in your template you can assign values to the context like this:

Linked

Create a file named `define_action.py` inside of the templatetags directory with the following code: