Template engines process template files, which provide an intermediate format between your Python code and a desired output format, such as HTML or PDF.
Template engines allow developers to generate a desired content type, 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 create.
<!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.
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.
Jinja, also known as "Jinja2", is a popular Python template engine written as an independent open source project, unlike some template engines that are provided as part of a larger web framework.
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's a whole lot more to learn about Jinja on the Jinja2 page.
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 is the default templating engine for the Pyramid web framework and has wide support as a replacement template engine for many other web frameworks.
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.
A Primer on Jinja Templating shows how to use the major parts of this fantastic template engine.
Template fragment gotchas is a collection of situations that can trip up a developer or designer when working with templates.
This template engines site contains a range of information from what templates engines are to listing more esoteric Python template engines.
Searching for a complete, step-by-step deployment walkthrough? Learn more about The Full Stack Python Guide to Deployments book.