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July 18, 2020 05:51 pm GMT

Jinja2 - Short introduction and Sample Apps

Hello Coders,

This article is a short introduction in Jinja2, a modern templating language used by Python programmers in frameworks like Flask, Bottle, optionally in Django from 1.8 version.

For those already familiar with Jinja2, and fast-runners, I'll drop some links below to some nice Jinja2 themes, provided as super simple Flask starters (no database or hard dependencies):

Demo Jinja2 Quick UI Kit - A modern UI kit converted to Jinja2
Demo Jinja2 Neumorphism UI - Just another free UI Kit
Demo Jinja2 AdminLTE - the famous OS dashboard design
Demo Jinja CoreUI - CoreUI dashboard design

All themes are provided by the AppSeed platform and the source code can be found on Github under the MIT License.

What is Jinja2

Jinja, is a Python template engine used to generate HTML or XML returned to the user via an HTTP response.

For those who have not been exposed to a templating language before, such languages essentially contain variables as well as some programming logic, which when evaluated (or rendered into HTML) are replaced with actual values.

Why do we need Jinja?

Sandboxed Execution - It provides a protected framework for automation of testing programs, whose behavior is unknown and must be investigated.

HTML Escaping Jinja 2 has a powerful automatic HTML Escaping, which helps to prevent Cross-site Scripting (XSS Attack). There are special characters like >,<,&, etc. which carry special meanings in the templates. So, if you want to use them as regular text in your documents then, replace them with entities. Not doing so might lead to XSS-Attack.

Template Inheritance - This feature helps us to generate new pages starting from a base template that we inherit a common structure.

How to get Jinja2

To start playing with it, just open a terminal and type:

$ pip install jinja2

Jinja in action

Simple runtime replace

>>> from jinja2 import Template>>> t = Template("Hello {{ token }}!")>>> t.render(token="Jinja2")u'Hello Jinja2!'

The engine will replace the inner token with value Jinja2. This is quite useful when we use this block for different token values.

Lists iteration

In web development, we can have cases when a list should be displayed on the page: registered users, for instance, or a simple list of options. In Jinja, we can use a for structure as bellow:

# Define data structuremy_list=[0,1,2,3,4,5] # a simple list with integers

In Jinja, we can iterate with ease, using a for block:

...      <ul>        {% for n in my_list %}        <li>{{n}}</li>        {% endfor %}      </ul>...

Template Inheritance

Templates usually take advantage of inheritance, which includes a single base template that defines the basic structure of all subsequent child templates. You use the tags { extends } and { block } to implement inheritance.

Let's take a look at a real sample:

Parent HTML - saved as base.html

<html>  <head>    <title>My Jinja {% block title %}{% endblock %} </title>  </head>  <body>    <div class="container">      <h2>This is from the base template</h2>      <br>      { block content }{ endblock }      <br>    </div>  </body></html>

The Child template - saved as child.html

{ extends "base.html" }{ block title } MySample { endblock }{ block content }  Cool content here{ endblock }

When Jinja loads child.html, the { extends } block informs the engine to merge the base.html template with the content provided by child.html

{ block title } become MySample
{ block content } become Cool content here

Here is the final HTML generated by Jinja:

<html>  <head>    <title>My Jinja MySample</title>  </head>  <body>    <div class="container">      <h2>This is from the base template</h2>      <br>      Cool content here      <br>    </div>  </body></html>

This powerful feature helps us to build complex web apps with ease by using common pages and components to generated dynamic pages hydrated with real information loaded from the database or provided by users, for instance.