How does templating engine work? - template-engine

Question

Could you please explain how templating engine works, for example in django with
context = {'q' : 1, 'w' : 2, items : [3,4,5,6]}
{% for item in items %}
<p>{{ item }}</p>
{% endfor %}
to
<p>3</p>
<p>4</p>
<p>5</p>
<p>6</p>

Answer 1

We can create a level of abstraction by introducing a new term "template engine language", which is different for each template engine.
During the runtime, behind the scenes, before content being send to browser, the interpreter of template engine walkthrough the code following different semantic and syntax rules (defined by template engine language) and acording to these rules, different actions are performed. Here we can make analogy with any programming language.
Let me show you how the above example can be written in PHP, and this time code will be interpreted by PHP interpreter rather than interpreter of template engine:
<?php
// Default data
$context = array( 'q' => 1, 'w' => 2, 'items' => array(1, 2, 3, 4) );
// Now array keys can be accessed as variables
extract($context);
foreach( $items as $item ) {
echo '<p>' . $item . '</p>' . PHP_EOL;
}
?>
Live: http://codepad.org/mxzJcC9N

Answer 2

Template Engine in Django
The template engine has two phases:
parsing
rendering
The parsing stage takes the template string and produces something
that could be rendered. Consider the template string as source code,
the parsing tool could be either a programming language interpreter or
a programming language compiler. If the tool is an interpreter,
parsing produces a data structure, the rendering tool will walk
through the structure and produces the result text. The Django
template engine parsing tool is an interpreter. Otherwise, parsing
produces some executable code, the rendering tool does nothing but
executes the code and produces the result. The Jinja2, Mako and
Tornado template module are all using a compiler as parsing tool.