WSGI - Web Server Gateway Interface.

What is WSGI?

WSGI is specified in PEP 3333. The main goal of WSGI is to facilitate easy interconnection of servers and web frameworks/applications. WSGI defines a standerd API for web servers(ex: uWSGI, Twisted, Gunicorn) to connect and talk to web frameworks like Django, flask, pyramid. Server and framework which followed WSGI specs then it enables the use of any framework with any server for example you can use uWSGi, Gunicorn or Twisted with django with little configuraton.

Hello World! programm using WSGI interface:

helloworld_wsgi_app.py

from wsgiref.simple_server import make_server

def helloworld_app(environ, start_response):

    """Simple web application which return Hello World to browser"""

    status = '200 OK'

    response_headers = [('Content-type', 'text/plain')]

    start_response(status, response_headers)

    return [b"Hello World !"]

if __name__ == '__main__':

    # start the serveer

    server = make_server('', 5000, helloworld_app)

    server.serve_forever()

python helloworld_wsgi_app.py

Now open the http://localhost:5000 you can see "Hello World !" text in the browser.

helloworld WSGI App Explanation

The application object must accept two arguments environ and start_response.

environ: The environ parameter is a dictionary object which contains wsgi and other environment variables. some of usefull environ variables or REQUEST_METHOD(The HTTP request method, such as "GET" or "POST"), QUERY_STRING (URL after the "?") etc.

start_response: The start_response parameter is a callable accepting two required positional arguments( status , response_headers). statu is used to specify http status 200, 302, 404, 500 etc. response_headers are used to specify content-type, Cookie, Authorization.

Response: HTTP deals with bytes so we have to return list of byte strings (ex: b"Hello World !").

In above example i have used default WSGI implimentaion of server(make_server) to run helloworld_app.