Is there a Django template tag that lets me set a context variable?

Question

I want to be able to set variables in a template to string values. I wrote a tag, but it doesn't seem to change the context. The intended use is:

{% define "a string" as my_var %}

Update (solved):

class DefineNode(Node):
    def __init__(self, var, name):
        self.var = var
        self.name = name

    def __repr__(self):
        return "<DefineNode>"

    def render(self, context):
        context[self.name] = self.var
        return ''

@register.tag
def define(parser, token):
    """
    Adds a name to the context for referencing an arbitrarily defined string.

    For example:

        {% define "my_string" as my_string %}

    Now anywhere in the template:

        {{ my_string }}
    """
    bits = list(token.split_contents())
    if (len(bits) != 4 or bits[2] != "as") or \
        not (bits[1][0] in ('"', "'") and bits[1][-1] == bits[1][0]):
        raise TemplateSyntaxError("%r expected format is '\"string\" as name'" % bits[0])
    else:
        value = bits[1][1:-1]
    name = bits[3]
    return DefineNode(value, name)

Answer 1

Django already considered this particular case and provides the assigment tags, an special way of registering tags that set a variable in the context.

In this case, you don't need to care about retrieving, updating and saving the context. You simply do this:

@register.assignment_tag
def define(the_string):
  return the_string

And you can use it the very same way, but it is much cleaner:

{% define "a string" as my_var %}

This all the code you need.

EDIT: As Dirk Bergstrom pointed out, since version django 1.9 assignment_tag is deprecated. simple_tag is a perfect replacement.

@register.simple_tag
def define(the_string):
  return the_string

Answer 2

The answer is buried inside the more complex current_time example in the documentation.

Problem

You want to add a variable to the context. But you don't want to go back and add that variable to all the views which call all the templates which invoke the tag. You just want a tag which can add some data to the context wherever its wanted. I'm looking for this kind of thing when rendering those random distractions which get dropped into sidebars and aren't specifically related to the work of the main view, for example.

Method

To inject a variable to the context you need access to the context. To do that your custom tag will inject a node which added the data to the template context.

Example

This example adds a "coming_events" queryset to the context then loops over each result. It does that by declaring a custom tag which renders a node which adds a queryset to the context.

from django import template
from apps.events.models import Event
register = template.Library()

@register.tag
def coming_events(parser, token):
    return EventNode()

class EventNode(template.Node):
    def render(self, context):
        context['coming_events'] = Event.objects.all()
        return ''

You'd use it like this:

{% load events %}
{% coming_events %}
{% for event in coming_events %}
<div class="eventItem">
   <p>{{event.title}} {{event.data}}</p>
</div>
{% endfor %}

Extra Credit

If you're really keen to be able to name the variable arbitrarily eg {% coming_events as events %} then look closely at the example in the documentation and note how they split the token into what's before the ' as ' and what's after and use the latter part to name the context variable. You'd have to implement that.

Note that if I wound up putting the HTML for each event into its own dedicated template then I'd be better off just following the standard inclusion tag example in the documentation. This solution is suggested for when you want the data without any baggage.