I think I understand WSGI

Ed Singleton — Thu, 20 Mar 2008 16:33:07 +0000

I’ve been messing around with Python web frameworks for a quite a while now. Trying out lots of different ones and seeing which one feels like the best fit. As I’ve got closer and closer to the right one, I’ve also found that i am getting closer and closer to using WSGI.

WSGI seems excellent and could be great for Python. Frameworks tend to make things seem more difficult and mysterious than they really are. What has taken me a long time to learn is that WSGI is really very simple.

At it’s simplest there are three aspects to it:

You receive a dictionary with useful information in it (environ)
You receive a function to call that starts the response (start_response)
You return an iterator (ie a list) of strings (this is your page body)

Anything on top of that is just helpers. Helpers are great, but you don’t need a framework that hides what is really going on, when it’s as simple as that.

You can create a hello world app as simple as:

def app(environ, start_response):
    start_response('200 OK', [('content-type', 'text/html')])
    return ["Hello World"]

Obviously that just returns Hello World with no html. Which is bad. So:

def app(environ, start_response):
    start_response('200 OK', [('content-type', 'text/html')])
    begin_body = ""
    body = "Hello World"
    end_body = ""
    return [begin_body, body, end_body]

We don’t want to write begin_body and end_body every time, so lets write a function to handle this for us:

def put_in_body(args):
    begin_body = ""
    end_body = ""
    return [begin_body] + list(args) + [end_body]

Then we can just:

def app(environ, start_response):
    start_response('200 OK', [('content-type', 'text/html')])
    return put_in_body("Hello World")

We’re going to want to be able to do lots of different things, and we don’t want to write them all in the body of one function, so let’s write a dispatcher:

def dispatch(environ):
    # We need to get the path from the environ, and work out where to go.
    path_info = environ.get('PATH_INFO', '')
    if path_info == "/goodbye":
        return goodbye(environ)
    else:
        return hello_world(environ)

def hello_world(environ):
    return ["Hello World"]

def goodbye(environ):
    return ["Goodbye World"]

def app(environ, start_response):
    start_response('200 OK', [('content-type', 'text/html')])
    return put_in_body(dispatch(environ))

Now if you go to /goodbye it will say “Goodbye World”, otherwise it will say “Hello World”.

As you can see it is all very straightforward. It allows you to do things however you want. Adding more and more functionality is just a case of adding functions that returns strings, and telling the dispatcher when to call that function.

In a later post I’ll talk about threading. It turns out that’s easy as well.

If you want to run the code above, just easy_install paste and then add this to the bottom of your file (which I called wsgi_sample.py):

if name == 'main':
    from paste import httpserver
    httpserver.serve(app, host='127.0.0.1', port='8080')

Then you can do python wsgi_sample.py and visit http://localhost:8080 to see it working (don’t forget to visit http://localhost:8080/goodbye before you leave).

Singletoned » werkzeug

I think I understand WSGI