‘Hello World’ with WSGI

I’ve been seeing a lot of talk about WSGI (Web Server Gateway Interface) and its benefits over the last six months or so and I’ve been meaning to take a look — not least because of the potential to use wsgi middleware to make a nice front-controller for KForge.

First Stop

A quick google takes me to: http://www.wsgi.org/wsgi. I’m looking to just write the proverbial ‘hello world’ app at this stage. Most of the references are bit too high level (or complex) for me (though this one is an exception). So here I’m going to detail my experiences of familiarizing myself with wsgi by writing the classic ‘hello world’ app (if you looking to do something more sophisticated with wsgi check out a toolkit such as paste or pylons the framework built on top of paste).

Hello World

1. Install wsgiref

wsgiref is the wsgi reference implementation that is now part of python 2.5 standard library. If you are running python version less than 2.5 you will want to do:

  $ sudo easy_install wsgiref

2. Get a web server

We’ll use the wsgiref simple server as detailed in the docs (if you want to use a ‘proper’ webserver see the section below on making your wsgi app available via fastcgi). Create a python module, simpletest.py say, and insert:

  from wsgiref.simple_server import make_server, demo_app

  httpd = make_server('', 8000, demo_app)
  print "Serving HTTP on port 8000..."

  # Respond to requests until process is killed
  httpd.serve_forever()

  # Alternative: serve one request, then exit
  ##httpd.handle_request()

3. Run it

Start the server:

  $ python simpletest.py

Then visit http://localhost:8000/

Bingo! We’ve got our first working wsgi app (demo_app should output ‘Hello world!’ followed by a list of variable values).

4. Make our own Hello World app

We haven’t yet written anything ourselves — we’re just using the demo_app bundled with wsgiref. So change simpletest.py to be:

  def simple_app(environ, start_response):
      """Simplest possible application object""" 
      status = '200 OK'
      response_headers = [('Content-type','text/plain')]
      start_response(status, response_headers)
      return ['My Own Hello World!\n']

  from wsgiref.simple_server import make_server, demo_app

  httpd = make_server('', 8000, simple_app)
  print "Serving HTTP on port 8000..."

  # Respond to requests until process is killed
  httpd.serve_forever()

Run this and visit http://localhost:8000/ and you should see a blank page containing ‘My Own Hello World!’.

5. Using a Class

Finally for completeness here’s the same application but done as a class:

  class SimpleApp:
      """Produce the same output, but using a class
      """
      def __init__(self, environ, start_response):
          self.environ = environ
          self.start = start_response

      def __iter__(self):
          status = '200 OK'
          response_headers = [('Content-type','text/plain')]
          self.start(status, response_headers)
          yield 'My Own Hello world!\n'

  from wsgiref.simple_server import make_server, demo_app

  # httpd = make_server('', 8000, simple_app)
  # the same but using a class
  httpd = make_server('', 8000, SimpleApp)

  print "Serving HTTP on port 8000..."

  # Respond to requests until process is killed
  httpd.serve_forever()

Serving an WSGI App via FastCGI

This section explains how to serve your WSGI app via FastCGI (other methods using scgi or even cgi take an almost identical approach).

1. Install a fastcgi interface to wsgi:

Use flup which provides a fastcgi and scgi interface to wsgi:

  $ sudo easy_install flup

2. Install a simple standalone fastcgi implementation:

1. Download http://www.saddi.com/software/py-lib/py-lib/fcgi.py
2. Install this somewhere you can import it as import fcgi

3. Attach your wsgi application to this fcgi server

Create a python file (server.fcgi) and paste in the following:

  #!/usr/bin/env python
  from myapplication import app # Assume app is your WSGI application object
  from fcgi import WSGIServer
  WSGIServer(app).run()

Now you can just point your webserver at this file (make sure you’ve configured it to handle .fcgi files using fastcgi) and your app is available via fastcgi.

References