The WSGI application interface is implemented as a callable object: a function, a method, a class or an instance with a __call__ method. That callable

• must accept two positional parameters:
  • A dictionary containing CGI like variables; and
  • a callback function that will be used by the application to send HTTP status code/message and HTTP headers to the server.
• and must return the response body to the server as strings wrapped in an iterable.

The application skeleton:

# This is our application object. It could have any name,
# except when using mod_wsgi where it must be "application"
def application( # It accepts two arguments:
      # environ points to a dictionary containing CGI like environment variables
      # which is filled by the server for each received request from the client
      environ,
      # start_response is a callback function supplied by the server
      # which will be used to send the HTTP status and headers to the server
      start_response):

   # build the response body possibly using the environ dictionary
   response_body = 'The request method was %s' % environ['REQUEST_METHOD']

   # HTTP response code and message
   status = '200 OK'

   # These are HTTP headers expected by the client.
   # They must be wrapped as a list of tupled pairs:
   # [(Header name, Header value)].
   response_headers = [('Content-Type', 'text/plain'),
                       ('Content-Length', str(len(response_body)))]

   # Send them to the server using the supplied function
   start_response(status, response_headers)

   # Return the response body.
   # Notice it is wrapped in a list although it could be any iterable.
   return [response_body]

This code will not run because it lacks the server instantiation step. Next page will show how to do it.