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Tips and Tricks
===============
.. highlight:: html+jinja
This part of the documentation shows some tips and tricks for Jinja2
templates.
.. _null-master-fallback:
Null-Master Fallback
--------------------
Jinja2 supports dynamic inheritance and does not distinguish between parent
and child template as long as no `extends` tag is visited. While this leads
to the surprising behavior that everything before the first `extends` tag
including whitespace is printed out instead of being ignored, it can be used
for a neat trick.
Usually child templates extend from one template that adds a basic HTML
skeleton. However it's possible to put the `extends` tag into an `if` tag to
only extend from the layout template if the `standalone` variable evaluates
to false which it does per default if it's not defined. Additionally a very
basic skeleton is added to the file so that if it's indeed rendered with
`standalone` set to `True` a very basic HTML skeleton is added::
{% if not standalone %}{% extends 'master.html' %}{% endif -%}
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<title>{% block title %}The Page Title{% endblock %}</title>
<link rel="stylesheet" href="style.css" type="text/css">
{% block body %}
<p>This is the page body.</p>
{% endblock %}
Alternating Rows
----------------
If you want to have different styles for each row of a table or
list you can use the `cycle` method on the `loop` object::
<ul>
{% for row in rows %}
<li class="{{ loop.cycle('odd', 'even') }}">{{ row }}</li>
{% endfor %}
</ul>
`cycle` can take an unlimited amount of strings. Each time this
tag is encountered the next item from the list is rendered.
Highlighting Active Menu Items
------------------------------
Often you want to have a navigation bar with an active navigation
item. This is really simple to achieve. Because assignments outside
of `block`\s in child templates are global and executed before the layout
template is evaluated it's possible to define the active menu item in the
child template::
{% extends "layout.html" %}
{% set active_page = "index" %}
The layout template can then access `active_page`. Additionally it makes
sense to define a default for that variable::
{% set navigation_bar = [
('/', 'index', 'Index'),
('/downloads/', 'downloads', 'Downloads'),
('/about/', 'about', 'About')
] -%}
{% set active_page = active_page|default('index') -%}
...
<ul id="navigation">
{% for href, id, caption in navigation_bar %}
<li{% if id == active_page %} class="active"{% endif
%}><a href="{{ href|e }}">{{ caption|e }}</a></li>
{% endfor %}
</ul>
...
.. _accessing-the-parent-loop:
Accessing the parent Loop
-------------------------
The special `loop` variable always points to the innermost loop. If it's
desired to have access to an outer loop it's possible to alias it::
<table>
{% for row in table %}
<tr>
{% set rowloop = loop %}
{% for cell in row %}
<td id="cell-{{ rowloop.index }}-{{ loop.index }}">{{ cell }}</td>
{% endfor %}
</tr>
{% endfor %}
</table>