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<div class="section" id="tips-and-tricks">
<h1>Tips and Tricks<a class="headerlink" href="#tips-and-tricks" title="Permalink to this headline">¶</a></h1>
<p>This part of the documentation shows some tips and tricks for Jinja2
templates.</p>
<div class="section" id="null-master-fallback">
<span id="id1"></span><h2>Null-Master Fallback<a class="headerlink" href="#null-master-fallback" title="Permalink to this headline">¶</a></h2>
<p>Jinja2 supports dynamic inheritance and does not distinguish between parent
and child template as long as no <cite>extends</cite> tag is visited. While this leads
to the surprising behavior that everything before the first <cite>extends</cite> tag
including whitespace is printed out instead of being igored, it can be used
for a neat trick.</p>
<p>Usually child templates extend from one template that adds a basic HTML
skeleton. However it’s possible put the <cite>extends</cite> tag into an <cite>if</cite> tag to
only extend from the layout template if the <cite>standalone</cite> 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
<cite>standalone</cite> set to <cite>True</cite> a very basic HTML skeleton is added:</p>
<div class="highlight-html+jinja"><div class="highlight"><pre><span class="cp">{%</span> <span class="k">if</span> <span class="k">not</span> <span class="nv">standalone</span> <span class="cp">%}{%</span> <span class="k">extends</span> <span class="s1">'master.html'</span> <span class="cp">%}{%</span> <span class="k">endif</span> -<span class="cp">%}</span>
<span class="cp"><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"></span>
<span class="nt"><title></span><span class="cp">{%</span> <span class="k">block</span> <span class="nv">title</span> <span class="cp">%}</span>The Page Title<span class="cp">{%</span> <span class="k">endblock</span> <span class="cp">%}</span><span class="nt"></title></span>
<span class="nt"><link</span> <span class="na">rel=</span><span class="s">"stylesheet"</span> <span class="na">href=</span><span class="s">"style.css"</span> <span class="na">type=</span><span class="s">"text/css"</span><span class="nt">></span>
<span class="cp">{%</span> <span class="k">block</span> <span class="nv">body</span> <span class="cp">%}</span>
<span class="nt"><p></span>This is the page body.<span class="nt"></p></span>
<span class="cp">{%</span> <span class="k">endblock</span> <span class="cp">%}</span>
</pre></div>
</div>
</div>
<div class="section" id="alternating-rows">
<h2>Alternating Rows<a class="headerlink" href="#alternating-rows" title="Permalink to this headline">¶</a></h2>
<p>If you want to have different styles for each row of a table or
list you can use the <cite>cycle</cite> method on the <cite>loop</cite> object:</p>
<div class="highlight-html+jinja"><div class="highlight"><pre><span class="nt"><ul></span>
<span class="cp">{%</span> <span class="k">for</span> <span class="nv">row</span> <span class="k">in</span> <span class="nv">rows</span> <span class="cp">%}</span>
<span class="nt"><li</span> <span class="na">class=</span><span class="s">"</span><span class="cp">{{</span> <span class="nb">loop</span><span class="nv">.cycle</span><span class="o">(</span><span class="s1">'odd'</span><span class="o">,</span> <span class="s1">'even'</span><span class="o">)</span> <span class="cp">}}</span><span class="s">"</span><span class="nt">></span><span class="cp">{{</span> <span class="nv">row</span> <span class="cp">}}</span><span class="nt"></li></span>
<span class="cp">{%</span> <span class="k">endfor</span> <span class="cp">%}</span>
<span class="nt"></ul></span>
</pre></div>
</div>
<p><cite>cycle</cite> can take an unlimited amount of strings. Each time this
tag is encountered the next item from the list is rendered.</p>
</div>
<div class="section" id="highlighting-active-menu-items">
<h2>Highlighting Active Menu Items<a class="headerlink" href="#highlighting-active-menu-items" title="Permalink to this headline">¶</a></h2>
<p>Often you want to have a navigation bar with an active navigation
item. This is really simple to achieve. Because assignments outside
of <cite>block</cite>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:</p>
<div class="highlight-html+jinja"><div class="highlight"><pre><span class="cp">{%</span> <span class="k">extends</span> <span class="s2">"layout.html"</span> <span class="cp">%}</span>
<span class="cp">{%</span> <span class="k">set</span> <span class="nv">active_page</span> <span class="o">=</span> <span class="s2">"index"</span> <span class="cp">%}</span>
</pre></div>
</div>
<p>The layout template can then access <cite>active_page</cite>. Additionally it makes
sense to defined a default for that variable:</p>
<div class="highlight-html+jinja"><div class="highlight"><pre><span class="cp">{%</span> <span class="k">set</span> <span class="nv">navigation_bar</span> <span class="o">=</span> <span class="o">[</span>
<span class="o">(</span><span class="s1">'/'</span><span class="o">,</span> <span class="s1">'index'</span><span class="o">,</span> <span class="s1">'Index'</span><span class="o">),</span>
<span class="o">(</span><span class="s1">'/downloads/'</span><span class="o">,</span> <span class="s1">'downloads'</span><span class="o">,</span> <span class="s1">'Downloads'</span><span class="o">),</span>
<span class="o">(</span><span class="s1">'/about/'</span><span class="o">,</span> <span class="s1">'about'</span><span class="o">,</span> <span class="s1">'About'</span><span class="o">)</span>
<span class="o">]</span> -<span class="cp">%}</span>
<span class="cp">{%</span> <span class="k">set</span> <span class="nv">active_page</span> <span class="o">=</span> <span class="nv">active_page</span><span class="o">|</span><span class="nf">default</span><span class="o">(</span><span class="s1">'index'</span><span class="o">)</span> -<span class="cp">%}</span>
...
<span class="nt"><ul</span> <span class="na">id=</span><span class="s">"navigation"</span><span class="nt">></span>
<span class="cp">{%</span> <span class="k">for</span> <span class="nv">href</span><span class="o">,</span> <span class="nv">id</span><span class="o">,</span> <span class="nv">caption</span> <span class="k">in</span> <span class="nv">navigation_bar</span> <span class="cp">%}</span>
<span class="nt"><li</span><span class="cp">{%</span> <span class="k">if</span> <span class="nv">id</span> <span class="o">==</span> <span class="nv">active_page</span> <span class="cp">%}</span> <span class="na">class=</span><span class="s">"active"</span><span class="cp">{%</span> <span class="k">endif</span>
<span class="cp">%}</span><span class="nt">><a</span> <span class="na">href=</span><span class="s">"</span><span class="cp">{{</span> <span class="nv">href</span><span class="o">|</span><span class="nf">e</span> <span class="cp">}}</span><span class="s">"</span><span class="nt">></span><span class="cp">{{</span> <span class="nv">caption</span><span class="o">|</span><span class="nf">e</span> <span class="cp">}}</span><span class="nt"></a></li></span>
<span class="cp">{%</span> <span class="k">endfor</span> <span class="cp">%}</span>
<span class="nt"></ul></span>
...
</pre></div>
</div>
</div>
<div class="section" id="accessing-the-parent-loop">
<span id="id2"></span><h2>Accessing the parent Loop<a class="headerlink" href="#accessing-the-parent-loop" title="Permalink to this headline">¶</a></h2>
<p>The special <cite>loop</cite> variable always points to the innermost loop. If it’s
desired to have access to an outer loop it’s possible to alias it:</p>
<div class="highlight-html+jinja"><div class="highlight"><pre><span class="nt"><table></span>
<span class="cp">{%</span> <span class="k">for</span> <span class="nv">row</span> <span class="k">in</span> <span class="nv">table</span> <span class="cp">%}</span>
<span class="nt"><tr></span>
<span class="cp">{%</span> <span class="k">set</span> <span class="nv">rowloop</span> <span class="o">=</span> <span class="nb">loop</span> <span class="cp">%}</span>
<span class="cp">{%</span> <span class="k">for</span> <span class="nv">cell</span> <span class="k">in</span> <span class="nv">row</span> <span class="cp">%}</span>
<span class="nt"><td</span> <span class="na">id=</span><span class="s">"cell-</span><span class="cp">{{</span> <span class="nv">rowloop.index</span> <span class="cp">}}</span><span class="s">-</span><span class="cp">{{</span> <span class="nb">loop</span><span class="nv">.index</span> <span class="cp">}}</span><span class="nt">></span><span class="cp">{{</span> <span class="nv">cell</span> <span class="cp">}}</span><span class="nt"></td></span>
<span class="cp">{%</span> <span class="k">endfor</span> <span class="cp">%}</span>
<span class="nt"></tr></span>
<span class="cp">{%</span> <span class="k">endfor</span> <span class="cp">%}</span>
<span class="nt"></table></span>
</pre></div>
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<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Tips and Tricks</a><ul>
<li><a class="reference internal" href="#null-master-fallback">Null-Master Fallback</a></li>
<li><a class="reference internal" href="#alternating-rows">Alternating Rows</a></li>
<li><a class="reference internal" href="#highlighting-active-menu-items">Highlighting Active Menu Items</a></li>
<li><a class="reference internal" href="#accessing-the-parent-loop">Accessing the parent Loop</a></li>
</ul>
</li>
</ul>
<h3>Related Topics</h3>
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<li>Previous: <a href="switching.html" title="previous chapter">Switching from other Template Engines</a></li>
<li>Next: <a href="faq.html" title="next chapter">Frequently Asked Questions</a></li>
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