Web Applications

As a powerful scripting language adapted to both fast prototyping and bigger projects, Python is widely used in web application development.



The Web Server Gateway Interface (or “WSGI” for short) is a standard interface between web servers and Python web application frameworks. By standardizing behavior and communication between web servers and Python web frameworks, WSGI makes it possible to write portable Python web code that can be deployed in any WSGI-compliant web server. WSGI is documented in PEP 3333.


Broadly speaking, a web framework consists of a set of libraries and a main handler within which you can build custom code to implement a web application (i.e. an interactive web site). Most web frameworks include patterns and utilities to accomplish at least the following:

URL Routing
Matches an incoming HTTP request to a particular piece of Python code to be invoked
Request and Response Objects
Encapsulate the information received from or sent to a user’s browser
Template Engine
Allows for separating Python code implementing an application’s logic from the HTML (or other) output that it produces
Development Web Server
Runs an HTTP server on development machines to enable rapid development; often automatically reloads server-side code when files are updated


Django is a “batteries included” web application framework. By providing many utilities and patterns out of the box, Django aims to make it possible to build complex, database-backed web applications quickly, while encouraging best practices in code written using it.

Django has a large and active community, and many pre-built re-usable modules that can be incorporated into a new project as-is, or customized to fit your needs.

There are annual Django conferences in the United States and in Europe.


Flask is a “microframework” for Python. Rather than aiming to provide everything you could possibly need, Flask implements the most commonly-used core components of a web application framework, like URL routing, request and response objects, and templates. As a user of Flask, it is therefore up to you to choose and integrate other components you may need, such as database access or form generation and validation. For many popular modules, Extensions may already exist to suit your needs.

Support for flask can best be found in its mailing list. Just shoot an email to flask@librelist.com and reply to the confirmation email.


Werkzeug is not actually a real framework, but rather a very powerful set of tools for building web applications. It provides URL routing utilities, request and response objects and a basic development server. It is mostly used where users need more flexibility for their application than is commonly found in other web frameworks.

Support can be found on its mailing list.


Tornado is a scalable, non-blocking web server and web application framework with a relative simple usage. Tornado is known for its high performance. It was initially developed for friendfeed , a real time chat and blog system.

In the Jinja2 template engine example it is used to serve the rendered pages.


Pyramid lies somewhere between a big framework like Django and the microframeworks: It comes with a lot of libraries and functionality and can thus not be considered lightweight. On the other hand, it does not provide all the functionality Django does. Instead Pyramid brings basic support for most regular tasks and provides a great deal of extensibility. Additionally, Pyramid has a huge focus on complete documentation. As a little extra it comes with the Werkzeug Debugger which allows you to debug a running web application in the browser.

Support can also be found in the documentation.

Web Servers


Nginx (pronounced “engine-x”) is a web server and reverse-proxy for HTTP, SMTP and other protocols. It is known for its high performance, relative simplicity, and compatibility with many application servers (like WSGI servers). It also includes handy features like load-balancing, basic authentication, streaming, and others. Designed to serve high-load websites, Nginx is gradually becoming quite popular.

WSGI Servers

Stand-alone WSGI servers typically use less resources than traditional web servers and provide top performance [3].


Gunicorn (Green Unicorn) is a WSGI server used to serve Python applications. It is a Python interpretation of the Ruby Unicorn server. Unicorn is designed to be lightweight, easy to use, and uses many UNIX idioms. Gunicorn is not designed to face the internet – it was designed to run behind Nginx which buffers slow requests and takes care of other important considerations. A sample setup for Nginx + Gunicorn can be found in the Gunicorn help.


uWSGI is a full stack for building hosting services. In addition to process management, process monitoring, and other functionality, uWSGI acts as an application server for various programming languages and protocols - including Python and WSGI. uWSGI can either be run as a stand-alone web router, or be run behind a full web server (such as Nginx or Apache). In the latter case, a web server can configure uWSGI and an application’s operation over the uwsgi protocol. uWSGI’s web server support allows for dynamically configuring Python, passing environment variables and further tuning. For full details, see uWSGI magic variables.

Server Best Practices

The majority of self-hosted Python applications today are hosted with a WSGI server such as Gunicorn, either directly or behind a lightweight web server such as nginx.

The WSGI servers serve the Python applications while the web server handles tasks better suited for it such as static file serving, request routing, DDoS protection, and basic authentication.



Platform-as-a-Service (PaaS) is a type of cloud computing infrastructure which abstracts and manages infrastructure, routing, and scaling of web applications. When using a PaaS, application developers can focus on writing application code rather than needing to be concerned with deployment details.

Most PaaS services offer a command-line interface that developers can use to set up and interrogate configuration, and to deploy new releases of an application to the service.

PaaS services and their partners offer add-on functionality which is well integrated into the platform, such as database hosting, email services, logging, scheduled and background tasks, billing and payment, etc.


Heroku‘s Cedar stack offers first class support for Python 2.7 applications.

Heroku allows you to run as many Python web applications as you like, 24/7 and free of charge. Heroku is best described as a horizontal scaling platform. They start to charge you once you “scale” your application to run on more than one Dyno (abstracted servers) at a time.

Heroku maintains articles on using Python with Heroku as well as step-by-step instructions on how to set up your first application.


DotCloud supports WSGI applications and background/worker tasks natively on their platform. Web applications run Python version 2.6, use nginx and uWSGI, and allow custom configuration of both for advanced users.

DotCloud uses a custom command-line API client which can work with applications managed in git repositories or any other version control system.

DotCloud has a free plan with limited database size, and without extra services (caching…).

See the DotCloud documentation on Python for more information and help getting started.


Gondor is a PaaS specialized for deploying Django and Pinax applications. Gondor recommends Django version 1.6 and supports any WSGI application on Python version 2.7. Gondor can automatically configure your Django site if you use local_settings.py for site-specific configuration information.

Gondor has a guide on deploying Django projects.


Most WSGI applications are responding to HTTP requests to serve content in HTML or other markup languages. Instead of generating directly textual content from Python, the concept of separation of concerns advises us to use templates. A template engine manages a suite of template files, with a system of hierarchy and inclusion to avoid unnecessary repetition, and is in charge of rendering (generating) the actual content, filling the static content of the templates with the dynamic content generated by the application.

As template files are sometimes written by designers or front-end developers, it can be difficult to handle increasing complexity.

Some general good practices apply to the part of the application passing dynamic content to the template engine, and to the templates themselves.

  • Template files should be passed only the dynamic content that is needed for rendering the template. Avoid the temptation to pass additional content “just in case”: it is easier to add some missing variable when needed than to remove a likely unused variable later.
  • Many template engines allow for complex statements or assignments in the template itself, and many allow some Python code to be evaluated in the templates. This convenience can lead to uncontrolled increase in complexity, and often make it harder to find bugs.
  • It is often necessary to mix JavaScript templates with HTML templates. A sane approach to this design is to isolate the parts where the HTML template passes some variable content to the JavaScript code.


Jinja2 is a template engine which is similar to the Django template system with some extra features. It is a text-based template language and thus can be used to generate any markup. It allows customization of filters, tags, tests and globals, and unlike the template system implemented in the Django Framework, also allows calling functions. Jinja2 is released under the BSD license.

Here some important html tags in Jinja2:

{# This is a comment #}

{# The next tag is a variable output: #}

{# Tag for a block, can be replaced through inheritance with other html code #}
{% block head %}
<h1>This is the head!</h1>
{% endblock %}

{# Output of an array as an iteration #}
{% for item in list %}
<li>{{ item }}</li>
{% endfor %}

The next listings is an example of a web site in combination with the Tornado web server. Tornado is not very complicated to use.

# import Jinja2
from jinja2 import Environment, FileSystemLoader

# import Tornado
import tornado.ioloop
import tornado.web

# Load template file templates/site.html
TEMPLATE_FILE = "site.html"
templateLoader = FileSystemLoader( searchpath="templates/" )
templateEnv = Environment( loader=templateLoader )
template = templateEnv.get_template(TEMPLATE_FILE)

# List for famous movie rendering
movie_list = [[1,"The Hitchhiker's Guide to the Galaxy"],[2,"Back to future"],[3,"Matrix"]]

# template.render() returns a string which contains the rendered html
html_output = template.render(list=movie_list,
                        title="Here is my favorite movie list")

# Handler for main page
class MainHandler(tornado.web.RequestHandler):
    def get(self):
        # Returns rendered template string to the browser request

# Assign handler to the server root  (
application = tornado.web.Application([
    (r"/", MainHandler),
if __name__ == "__main__":
    # Setup the server

The base.html file can be used as base for all site pages which are for example implemented in the content block.

<html lang="en">
<html xmlns="http://www.w3.org/1999/xhtml">
    <link rel="stylesheet" href="style.css" />
    <title>{{title}} - My Webpage</title>
<div id="content">
    {# In the next line the content from the site.html template will be added #}
    {% block content %}{% endblock %}
<div id="footer">
    {% block footer %}
    &copy; Copyright 2013 by <a href="http://domain.invalid/">you</a>.
    {% endblock %}

The next listing is our site page (site.html) loaded in the Python app which extends base.html. The content block is automatically set into the corresponding block in the base.html page.

<!{% extends "base.html" %}
{% block content %}
    <p class="important">
    <div id="content">
        <p>{{ list_title }}</p>
             {% for item in list %}
             <li>{{ item[0]}} :  {{ item[1]}}</li>
             {% endfor %}
{% endblock %}


Chameleon Page Templates are an HTML/XML template engine implementation of the Template Attribute Language (TAL), TAL Expression Syntax (TALES), and Macro Expansion TAL (Metal) syntaxes.

Chameleon is available for Python 2.5 and up (including 3.x and pypy), and is commonly used by the Pyramid Framework.

Page Templates add within your document structure special element attributes and text markup. Using a set of simple language constructs, you control the document flow, element repetition, text replacement and translation. Because of the attribute-based syntax, unrendered page templates are valid HTML and can be viewed in a browser and even edited in WYSIWYG editors. This can make round-trip collaboration with designers and prototyping with static files in a browser easier.

The basic TAL language is simple enough to grasp from an example:

  <h1>Hello, <span tal:replace="context.name">World</span>!</h1>
      <tr tal:repeat="row 'apple', 'banana', 'pineapple'">
        <td tal:repeat="col 'juice', 'muffin', 'pie'">
           <span tal:replace="row.capitalize()" /> <span tal:replace="col" />

The <span tal:replace=”expression” /> pattern for text insertion is common enough that if you do not require strict validity in your unrendered templates, you can replace it with a more terse and readable syntax that uses the pattern ${expression}, as follows:

    <h1>Hello, ${world}!</h1>
      <tr tal:repeat="row 'apple', 'banana', 'pineapple'">
        <td tal:repeat="col 'juice', 'muffin', 'pie'">
           ${row.capitalize()} ${col}

But keep in mind that the full <span tal:replace=”expression”>Default Text</span> syntax also allows for default content in the unrendered template.


[1]The mod_python project is now officially dead
[2]mod_wsgi vs mod_python
[3]Benchmark of Python WSGI Servers