Template Engines

Template engines process template files, which provide an intermediate format between your Python code and a desired output format, such as HTML or PDF.

Why are template engines important?

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.

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

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 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

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.

Template engine resources

Do you want to learn about web frameworks, CSS or JavaScript next?

I want to learn how to code a Python web application using a framework.

My app runs but looks awful. How do I style the user interface?

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