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<?xml version="1.0" encoding="utf-8"?>
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<head>
<title>Tag Howto | Jinja Documentation</title>
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<div id="navigation">
<h3>Documentation</h3>
<ul>
<li><a href="index.html">back to index</a></li>
<li><a href="http://wsgiarea.pocoo.org/repos/jinja/trunk/docs/source/tag-dev.txt">view source online</a></li>
</ul>
<h3>Table of Contents</h3>
<ul>
<li><a href="#diving-in">Diving In</a></li>
<li><a href="#example">Example</a></li>
<li><a href="#nodes">Nodes</a></li>
<li><a href="#keywordnode">KeywordNode</a></li>
<li><a href="#variablenode">VariableNode</a></li>
<li><a href="#valuenode">ValueNode</a></li>
<li><a href="#choicenode">ChoiceNode</a></li>
<li><a href="#collectionnode">CollectionNode</a></li>
<li><a href="#one-way-parsing">One-Way Parsing</a></li>
<li><a href="#two-way-parsing">Two-Way Parsing</a></li>
</ul>
</div>
<h1>Tag Howto</h1>
<div id="page">
<p>Tags are much more complex than <a class="reference" href="./filter-dev.html">filters</a>, because with tags you can do
everything.</p>
<p>While filters only extend the behavior of tags like <tt class="docutils literal"><span class="pre">print</span></tt> and <tt class="docutils literal"><span class="pre">filter</span></tt>,
the whole Jinja core uses tags to handle loops, conditions...</p>
<p>When Jinja compiles a template, it splits the raw template text into
''nodes''. Each node is an instance of <tt class="docutils literal"><span class="pre">jinja.nodes.Node</span></tt> and has a
<tt class="docutils literal"><span class="pre">render(context)</span></tt> method. A compiled template is, simply, a list of <tt class="docutils literal"><span class="pre">Node</span></tt>
objects. When you call <tt class="docutils literal"><span class="pre">render()</span></tt> on a compiled template object, the template
calls <tt class="docutils literal"><span class="pre">render()</span></tt> on each <tt class="docutils literal"><span class="pre">Node</span></tt> in its node list, with the given context.
The results are all concatenated together to form the output of the template.</p>
<p>When Jinja encounters a <tt class="docutils literal"><span class="pre">BlockToken</span></tt> in the template it looks at the defined
library and let it parse the token content (e.g. <tt class="docutils literal"><span class="pre">for</span> <span class="pre">item</span> <span class="pre">in</span> <span class="pre">sequence</span></tt>).</p>
<p>When no library is defined it uses the standard library <tt class="docutils literal"><span class="pre">stdlib</span></tt>.</p>
<div class="section">
<h2><a id="diving-in" name="diving-in">Diving In</a></h2>
<p>Each Tag object has to look at least like this:</p>
<pre class="literal-block">
from jinja.lib import stdlib
from jinja.nodes import *
class MyTag(Node):
rules = {}
def __init__(self, parser, matched_tag, handler_args, stack):
pass
def render(self, context):
return ''
stdlib.register_tag(MyTag)
</pre>
<p><tt class="docutils literal"><span class="pre">rules</span></tt> is a dict of parser instructions:</p>
<pre class="literal-block">
rules = {
'default': [KeywordNode('mytag')],
'witharg': [KeywordNode('mytag'), ChoiceNode()]
}
</pre>
<p>This rule definition would match all <tt class="docutils literal"><span class="pre">{%</span> <span class="pre">mytag</span> <span class="pre">%}</span></tt> and <tt class="docutils literal"><span class="pre">{%</span> <span class="pre">mytag</span> <span class="pre">arg</span> <span class="pre">%}</span></tt>
tags.</p>
<p>The <tt class="docutils literal"><span class="pre">__init__</span></tt> method gets called on tag creation. When you're using a cached
loader it will save the tag in the state of leaving the <tt class="docutils literal"><span class="pre">__init__</span></tt> method.</p>
<p>The arguments are these:</p>
<ul class="simple">
<li><strong>parser</strong> - a template parser instance which can be used to parse parts
of the template.</li>
<li><strong>matched_tag</strong> - a string containing the name of the matched rule.
(e.g. <tt class="docutils literal"><span class="pre">witharg</span></tt> or <tt class="docutils literal"><span class="pre">default</span></tt> in the example above)</li>
<li><strong>handler_args</strong> - the list of argument nodes.</li>
<li><strong>stack</strong> - list of piped filters</li>
</ul>
</div>
<div class="section">
<h2><a id="example" name="example">Example</a></h2>
<p>To understand this here is the defintion of the <tt class="docutils literal"><span class="pre">print</span></tt> tag:</p>
<pre class="literal-block">
class VariableTag(Node):
rules = {
'default': [KeywordNode('print'), ChoiceNode()]
}
def __init__(self, parser, matched_tag, handler_args, stack):
self._variable = handler_args[1]
self._filters = [(f, args[1:][0]) for f, _, args in stack]
def findnodes(self):
yield self._variable
def render(self, context):
if not self._filters:
return self._variable.render(context)
var = self._variable.resolve(context)
for f, args in self._filters:
var = f(var, *[arg.resolve(context) for arg in args])
return var
stdlib.register_tag(VariableTag)
</pre>
<p>The <tt class="docutils literal"><span class="pre">rules</span></tt> dict defines a rule matching all <tt class="docutils literal"><span class="pre">{%</span> <span class="pre">print</span> <span class="pre">variable</span> <span class="pre">%}</span></tt>.</p>
<p>A <tt class="docutils literal"><span class="pre">ChoiceNode</span></tt> matches per default all variables and string/integer
constants.</p>
<p>The <tt class="docutils literal"><span class="pre">__init__</span></tt> methods saves the variable node and the list of filters
in the Tag.</p>
<p>The <tt class="docutils literal"><span class="pre">findnodes</span></tt> method has to return a iterable of all nodes defined
in the <tt class="docutils literal"><span class="pre">Tag</span></tt>.</p>
<p>In the render method the <tt class="docutils literal"><span class="pre">VariableTag</span></tt> returns a parsed content of the
variable by applying all filters.</p>
</div>
<div class="section">
<h2><a id="nodes" name="nodes">Nodes</a></h2>
<p>Jinja shipps a number of nodes the parser can match. All this nodes are
defined in the <tt class="docutils literal"><span class="pre">jinja.nodes</span></tt> module.</p>
<div class="section">
<h3><a id="keywordnode" name="keywordnode">KeywordNode</a></h3>
<p>A keyword node matches against a constant keyword value. You can compare
Keywords with strings which simplyfies the postprocessing:</p>
<pre class="literal-block">
if my_keyword_node == "foo":
...
else:
...
</pre>
<p>It isn't possible to resolve or render keyworde nodes.</p>
</div>
<div class="section">
<h3><a id="variablenode" name="variablenode">VariableNode</a></h3>
<p>A variable node matches all possible variables by saving the name.
Variable nodes provide a <tt class="docutils literal"><span class="pre">define</span></tt> method for updating their value:</p>
<pre class="literal-block">
varnode.define(context, 'new value')
</pre>
<p>You can get the value of a <tt class="docutils literal"><span class="pre">VariableNode</span></tt> using resolve:</p>
<pre class="literal-block">
value = varnode.resolve(context)
</pre>
<p>variable nodes do also provide a render method which acts like the resolve
method but returns a string.</p>
</div>
<div class="section">
<h3><a id="valuenode" name="valuenode">ValueNode</a></h3>
<p>Value nodes behaves like variable nodes but match strings, integers, boolean
values and "none".</p>
<p>It provides the same functionallity like the <tt class="docutils literal"><span class="pre">VariableNode</span></tt>, but resolve
can also get called without the context which allows you to fetch the constant
value inside the <tt class="docutils literal"><span class="pre">__init__</span></tt> method of a tag.</p>
</div>
<div class="section">
<h3><a id="choicenode" name="choicenode">ChoiceNode</a></h3>
<p>A choice node matches more than one one nodetype:</p>
<pre class="literal-block">
ChoiceNode(Node1(), Node2())
</pre>
<p>When not given any arguments it will match eigther one <tt class="docutils literal"><span class="pre">VariableNode</span></tt> or
<tt class="docutils literal"><span class="pre">ValueNode</span></tt>.</p>
</div>
<div class="section">
<h3><a id="collectionnode" name="collectionnode">CollectionNode</a></h3>
<p>A collection node matches an unlimited number of Nodes:</p>
<pre class="literal-block">
CollectionNode(Node1(), Node2())
</pre>
<p>When not given any arguments it will match all variable and/or value nodes.</p>
</div>
</div>
<div class="section">
<h2><a id="one-way-parsing" name="one-way-parsing">One-Way Parsing</a></h2>
<p>One way parsing is very basic:</p>
<pre class="literal-block">
{% mynode %}
...
{% endmynode %}
</pre>
<p>You can fetch the <tt class="docutils literal"><span class="pre">body</span></tt> between those two tags inside the <tt class="docutils literal"><span class="pre">__init__</span></tt>
method of you <tt class="docutils literal"><span class="pre">MyTag</span></tt> class:</p>
<pre class="literal-block">
self._body = parser.subparse('endmynode')
</pre>
<p>This will store all the nodes from <tt class="docutils literal"><span class="pre">mynode</span></tt> to <tt class="docutils literal"><span class="pre">endmynode</span></tt> which you can
render using <tt class="docutils literal"><span class="pre">self._body.render(context)</span></tt>.</p>
</div>
<div class="section">
<h2><a id="two-way-parsing" name="two-way-parsing">Two-Way Parsing</a></h2>
<p>Two way parsing is a bit more complicated:</p>
<pre class="literal-block">
{% mynode %}
...
{% switchmynode %}
...
{% endmynode %}
</pre>
<p>But it would also match:</p>
<pre class="literal-block">
{% mynode %}
...
{% endmynode %}
</pre>
<p>Parsing this would result in two bodies:</p>
<pre class="literal-block">
self._body_one, self._body_two = parser.forkparse('switchmynode', 'endmynode')
</pre>
<p>When the parser doesn't find the <tt class="docutils literal"><span class="pre">switchmynode</span></tt> tag it will returns an
empty <tt class="docutils literal"><span class="pre">NodeList</span></tt> for <tt class="docutils literal"><span class="pre">self._body_two</span></tt>.</p>
<p>For more informations have a look at the <a class="reference" href="http://wsgiarea.pocoo.org/trac/browser/jinja/trunk/jinja/tags.py">Tags</a> module in the jinja source.</p>
</div>
</div>
</body>
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