Template Engines

Template engines take in tokenized strings and produce rendered strings with values in place of the tokens as output. Templates are typically used as an intermediate format written by developers to programmatically produce one or more desired output formats, commonly HTML, XML or PDF.

Why are template engines important?

Template engines allow developers to generate desired content types, such as HTML, while using some of the data and programming constructs such as conditionals and for loops to manipulate the output. Template files that are created by developers and then processed by the template engine consist of prewritten markup and template tag blocks where data is inserted.

For example, look at the first ten source lines of HTML of this webpage:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="author" content="Matt Makai">
  <meta name="description" content="Template engines provide programmatic output of formatted content such as HTML, XML or PDF.">
  <link rel="shortcut icon" href="//static.fullstackpython.com/fsp-fav.png">

Every one of the HTML lines above is standard for each page on Full Stack Python, with the exception of the <meta name="description"... line which provides a unique short description of what the individual page contains.

The base.html Jinja template used to generate Full Stack Python allows every page on the site to have consistent HTML but dynamically generate the pieces that need to change between pages when the static site generator executes. The below code from the base.html template shows that the meta description is up to child templates to generate.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="author" content="Matt Makai">
  {% block meta_header %}{% endblock %}
  <link rel="shortcut icon" href="//static.fullstackpython.com/fsp-fav.png">

In a typical WSGI application, the template engine would generate the HTML output response when an HTTP request comes in for a particular URL.

Python template engines

There are several popular Python template engines. A template engine implementation will fall somewhere on the spectrum between allowing arbitrary code execution and granting only a limited set of capabilities via template tags. A rough visual of the code in template spectrum can be seen below for four of the major Python template engines.

Spectrum between no logic in templates and the ability to run arbitrary code.

Jinja (Jinja2)

Jinja, also known and referred to as "Jinja2", is a popular Python template engine written as a self-contained open source project. Some template engines, such as Django templates are provided as part of a larger web framework, which can make them difficult to reuse in projects outside their coupled library.

Major Python open source applications such as the configuration management tools Ansible and SaltStack as well as the static site generator Pelican use the Jinja template engine by default for generating output files.

There is a whole lot more to learn about Jinja on the Jinja2 page.

Django templating

Django comes with its own template engine in addition to supporting (as of Django 1.9) drop-in replacement with other template engines such as Jinja.

Mako template engine

Mako was the default templating engine for the Pylons web framework and is one of many template engines supported by Pyramid. Mako has wide support as a replacement template engine for many other web frameworks as well.

Other Python template engine implementations

There are numerous Python template engine implementations that range from weekend hacks to actively developed mature libraries. These template engines are listed alphabetically:

Chameleon is an HTML and XML template engine that supports both Python 2 and 3.
Cheetah
Diazo
evoque
Genshi
Juno
Myghty
pyratemp
pystache

Template engine implementation comparisons

There are many Python template engine implementations in addition to the ones listed above. These resources can help you select a Python template engine implementation that works well for your project.

Python Web Templating Battle compares and contrasts Django templates, Chameleon, Jinja, Diazo and Mako, with examples for each engine.
This template engines site contains a range of information from what templates engines are to listing more esoteric Python template engines.
Python Template Engine Comparison is an older but still relevant post by the creator of Jinja that explains why and how he switches between Mako, Jinja and Genshi for various projects he works on.
Python Web Frameworks: What are the advantages and disadvantages of using Mako vs. Jinja2? has some good answers from developers on Quora about using Mako compared with Jinja2.

Template engine resources

Template engines are often used with web frameworks a black box where input goes in, and rendered text magically appears out the other side. However, when something unexpected returns from a template engine it is useful to know how they work to aid your debugging. The following resources examine existing template engine design as well as how to build your own engine when that's necessary for your projects.

How a template engine works uses the template module in Tornado as an example to step through how a template engine produces output, from parsing the incoming string to rendering the final output.
A template engine in 500 lines or less is an article by Ned Batchelder provides a
template engine in 252 lines of Python that can be used to understand how template engines work under the cover.
The world's simplest Python template engine shows how the format() function can implement a simple template engine with conditionals, loops and method invocations.
When to use a Templating Engine (in Python)? is a Stack Overflow question with a useful answer on why and when to use an existing template engine.
Template Engines uses Jinja as an implementation example to explain the tasks that template engines can be used to perform.
Approach: Building a toy template engine in Python walks through how to create your own simple template engine in Python to understand the basics of how most template engines work.