Templating Engine

Motivation

A templating engine combines templates and data models to produce result documents. A good analogy is baking cookies. You have a cookie cutter(template) and your dough(the data model). You take your cookie cutter and, depending on the dough, you will stamp out cookies that are related but not identical.

Templating engines allows you to seperate logic from presentation in a program. By splitting these two concerns you are free to modify each piece independently. The split between logic and presentation is well explored and serves as the primary motivation behind the popular Model-View-Controller design pattern driving many web and mobile applications today.

In this practice problem, we will try to build a templating engine with a specialized syntax. Some templating engines allow you to execute normal python code, but that dangles the carrot of model data mutation a little too close for comfort.

Prerequisites

It is helpful to have some experience using a templating engine before you start this project. You can download one of the engines below to start experimenting. However, almost everybody has used a basic templating engine before! Python string formatting is just a simple templating engine.

name = <insert your name here>
"Hello %s, I hope you're well today" % name

Problem Statement

We will stick closely to the jinja2/django syntax because that is what people are most familiar with. If you're feeling adventerous, feel free to invent your own syntax.

The rendering of a template should be fairly easy. The template should take in a dictionary that provides all the data required to render it.

Here is an example of what our template should be able to do.

>>template = Template(open("our_template.tmpl", "r"))
>>data = {"name": "Eva", "age": 23, "apple_count": 5}
>>output = template.render(data_model=data)
>>print output

Hello Eva, what a fine age, 23, to be baking apple pies. You need 7 more apples until you have a dozen. Come back when you're ready!

>>data = {"name": "Eva", "age": 23, "apple_count": 12, "friends": ["Billy", "John", "Emily"]}
>>output = template.render(data_model=data)
>>print output

Hello Eva, what a fine age, 23, to be baking apple pies. It looks like you've got a round dozen. I'll just go preheat the oven now. 
After, you can call Billy, John, Emily to help us eat.

String Interpolation

This is just a direct substitution of the string.

"Hello {{name}}, what a fine age, {{age}}, to be baking apple pies"

Conditionals and Comparisons

Conditionals allow you to make presentation decisions based on your data.

{% if apple_count >= 12 %}
    ...
{% else %}
    ...

Calculations

Simple arithmetic should be supported within your template delimiters.

"You need {{12 - apple_count}} until you have a round dozen"

Loops

You should be able to loop through lists (and maybe even dictionaries!) to show collections of data.

"After, you can call {% for friend in friends %}{{friend}},{% endfor %} to help us eat."

Template Inheritance

Inheritance allows you to factor out common elements in your page. The template hierarchy acts similar to a class hierarchy.

base.templ:

<!doctype html>
<html>
    <head>
        {% block head %}{% endblock %}
    </head>
    <body>
        {% block body %}{% end block %}
    </body>
</html>

inheritance_eg.templ:

{% extends base.template %}

{% block head %}
<title>My first templated page!</title>
{% endblock %}

{% block body %}
<h1>Hello to the templated world!</h1>
{% endblock %}

You can probably imagine what the output but here it just for completeness.

>>template = Template(open("inheritance_eg.tmpl", "r"))
>>output = template.render()
>>print output

<!doctype html>
<html>
    <head>
        <title>My first templated page!</title>
    </head>
    <body>
        <h1>Hello to the templated world!</h1>
    </body>
</html>

That's it for your simple template engine! This is just scratching the surface, feel free to dig into production templating engines and see how far you can take it.

References

Toy Templating Engine - Alex Michael
Jinja - Armin Ronacher
Django Templates - Django Project
Pyratemp - Roland Koebler
Evoque - Mario Ruggier