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Scope of variables in template blocks

07 Apr 6

Scope of variables in template blocks

The Django documentation gives explicit details on how you can set a variable in the context using a custom template tag.

class CurrentTimeNode3(template.Node):
    def __init__(self, format_string, var_name):
        self.format_string = format_string
        self.var_name = var_name
    def render(self, context):
        context[self.var_name] = datetime.datetime.now().strftime(self.format_string)
        return ''

Where can I use that variable?

Of course you can use in the template, but the scope depends on the templates structure. Let's take as example a template like this:

{% extends 'site_base.html' %}

{% load my_custom_tags %}

{% block head %}
    {% get_current_time "%Y-%M-%d %I:%M %p" as my_current_time %}
    The current time is {{ my_current_time }}.
{% endblock %}

This will have the expected output, in the head block of the site_base.html template will display the time.

The exception raises when you have multiple blocks where you want to use that variable:

{% extends 'site_base.html' %}

{% load my_custom_tags %}

{% block head %}
    {% get_current_time "%Y-%M-%d %I:%M %p" as my_current_time %}
    The current time is {{ my_current_time }}.
{% endblock %}

{% block content %}
    The current time is {{ my_current_time }}.
{% endblock %}

This usage will not display the time in the content block or if you set a list that you will iterate over, it will raise Caught an exception while rendering: Failed lookup for key [list] in u'[{\'block\': ...

Why is it a problem to call the template tag in both blocks?

Imagine you are doing some queries in that custom template tag, calling it in multiple blocks, increases your queries/page stat.

Why can't I use the variable in any block, since it is set in the context?

{% block %} pushes a new dictionary on the Context stack and pops it off when it reaches the matching {% endblock %}. This means any context value created while in the block has essentially gone out of scope on block exit.

Any solution?

A minor adjustment to set the variable in the main context

def render(self, context):
        context.dicts[0][self.var_name] = datetime.datetime.now().strftime(self.format_string)
        return ''

Category: Django

Discussion

Stefan Talpalaru on Apr 07, 2010 - 10:25 said:

How about using the custom tag outside any template block?
Liviu Chiributa on Apr 08, 2010 - 1:34 said:

Stefan, the django book says that "Any text outside of {% block %} tags in child templates will not be rendered.", it will not enter the Nodes render function.
Stefan Talpalaru on Apr 08, 2010 - 3:26 said:

Have you tested it?
Liviu Chiributa on Apr 08, 2010 - 5:07 said:

I tested like this:
1. in site_base.html i used it to instantiate the variable (outside of a block), it was accessible in the child template in any block
2. in the child template instantiate it outside of a block, the render function is not called.
JoeZ99 on Nov 22, 2010 - 14:42 said:

you just saved my life
Luis on Apr 18, 2011 - 13:50 said:

Nice post! Well written, easy to follow, concise and really useful for me. Thanks!

Scope of variables in template blocks