Application Interface

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

1. 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.

2. return the response body to the server as strings wrapped in an iterable.

The application skeleton:

# The application interface is a callable object
def application ( # It accepts two arguments:
    # environ points to a dictionary containing CGI like environment
    # variables which is populated by the server for each
    # received request from the client
    environ,
    # start_response is a callback function supplied by the server
    # which takes the HTTP status and headers as arguments
    start_response
):

    # Build the response body possibly
    # using the supplied environ dictionary
    response_body = 'Request method: %s' % environ['REQUEST_METHOD']

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

    # 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.