Introduction and Audience

This short tutorial is meant to teach you a little about WSGI, and as an example a bit about the architecture that Paste has enabled and encourages.

This isn't an introduction to all the parts of Paste -- in fact, we'll only use a few, and explain each part. This isn't to encourage everyone to go off and make their own framework (though honestly I wouldn't mind). The goal is that when you have finished reading this you feel more comfortable with some of the frameworks built using this architecture, and a little more secure that you will understand the internals if you look under the hood.

What is WSGI?

At its simplest WSGI is an interface between web servers and web applications. We'll explain the mechanics of WSGI below, but a higher level view is to say that WSGI lets code pass around web requests in a fairly formal way. But there's more! WSGI is more than just HTTP. It might seem like it is just barely more than HTTP, but that little bit is important:

• You pass around a CGI-like environment, which means data like REMOTE_USER (the logged-in username) can be securely passed about.
• A CGI-like environment can be passed around with more context -- specifically instead of just one path you two: SCRIPT_NAME (how we got here) and PATH_INFO (what we have left).
• You can -- and often should -- put your own extensions into the WSGI environment. This allows for callbacks, extra information, arbitrary Python objects, or whatever you want. These are things you can't put in custom HTTP headers.

This means that WSGI can be used not just between a web server an an application, but can be used at all levels for communication. This allows web applications to become more like libraries -- well encapsulated and reusable, but still with rich reusable functionality.

Writing a WSGI Application

The first part is about how to use WSGI at its most basic. You can read the spec, but I'll do a very brief summary:

• You will be writing a WSGI application. That's an object that responds to requests. An application is just a callable object (like a function) that takes two arguments: environ and start_response.
• The environment looks a lot like a CGI environment, with keys like REQUEST_METHOD, HTTP_HOST, etc.
• The environment also has some special keys like wsgi.input (the input stream, like the body of a POST request).
• start_response is a function that starts the response -- you give the status and headers here.
• Lastly the application returns an iterator with the body response (commonly this is just a list of strings, or just a list containing one string that is the entire body.)

So, here's a simple application:

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

Well... that's unsatisfying. Sure, you can imagine what it does, but you can't exactly point your web browser at it.

There's other cleaner ways to do this, but this tutorial isn't about clean it's about easy-to-understand. So just add this to the bottom of your file:

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

Now visit http://localhost:8080 and you should see your new app. If you want to understand how a WSGI server works, I'd recommend looking at the CGI WSGI server in the WSGI spec.

from paste.request import parse_formvars

def app(environ, start_response):
    fields = parse_formvars(environ)
    if environ['REQUEST_METHOD'] == 'POST':
        start_response('200 OK', [('content-type', 'text/html')])
        return ['Hello, ', fields['name'], '!']
    else:
        start_response('200 OK', [('content-type', 'text/html')])
        return ['<form method="POST">Name: <input type="text" '
                'name="name"><input type="submit"></form>']

Now For a Framework

Now, this probably feels a bit crude. After all, we're testing for things like REQUEST_METHOD to handle more than one thing, and it's unclear how you can have more than one page.

We want to build a framework, which is just a kind of generic application. In this tutorial we'll implement an object publisher, which is something you may have seen in Zope, Quixote, or CherryPy.

def __call__(self, environ, start_response):
    fields = parse_formvars(environ)
    obj = self.find_object(self.root, environ)
    response_body = obj(**fields.mixed())
    start_response('200 OK', [('content-type', 'text/html')])
    return [response_body]

def find_object(self, obj, environ):
    path_info = environ.get('PATH_INFO', '')
    if not path_info or path_info == '/':
        # We've arrived!
        return obj
    # PATH_INFO always starts with a /, so we'll get rid of it:
    path_info = path_info.lstrip('/')
    # Then split the path into the "next" chunk, and everything
    # after it ("rest"):
    parts = path_info.split('/', 1)
    next = parts[0]
    if len(parts) == 1:
        rest = ''
    else:
        rest = '/' + parts[1]
    # Hide private methods/attributes:
    assert not next.startswith('_')
    # Now we get the attribute; getattr(a, 'b') is equivalent
    # to a.b...
    next_obj = getattr(obj, next)
    # Now fix up SCRIPT_NAME and PATH_INFO...
    environ['SCRIPT_NAME'] += '/' + next
    environ['PATH_INFO'] = rest
    # and now parse the remaining part of the URL...
    return self.find_object(next_obj, environ)

from objectpub import ObjectPublisher

class Root(object):

    # The "index" method:
    def __call__(self):
        return '''
        <form action="welcome">
        Name: <input type="text" name="name">
        <input type="submit">
        </form>
        '''

    def welcome(self, name):
        return 'Hello %s!' % name

app = ObjectPublisher(Root())

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

# This is just a dictionary-like object that has case-
# insensitive keys:
from paste.response import HeaderDict

class Request(object):
    def __init__(self, environ):
        self.environ = environ
        self.fields = parse_formvars(environ)

class Response(object):
    def __init__(self):
        self.headers = HeaderDict(
            {'content-type': 'text/html'})

class ObjectPublisher(object):
    ...

    def __call__(self, environ, start_response):
        fields = parse_formvars(environ)
        obj = self.find_object(self.root, environ)
        response_body = obj(**fields.mixed())
        start_response('200 OK', [('content-type', 'text/html')])
        return [response_body]

import threading
webinfo = threading.local()

    def __call__(self, environ, start_response):
        webinfo.request = Request(environ)
        webinfo.response = Response()
        obj = self.find_object(self.root, environ)
        response_body = obj(**dict(webinfo.request.fields))
        start_response('200 OK', webinfo.response.headers.items())
        return [response_body]

class Root:
    def rss(self):
        webinfo.response.headers['content-type'] = 'text/xml'
        ...

WSGI Middleware

Middleware is where people get a little intimidated by WSGI and Paste.

What is middleware? Middleware is software that serves as an intermediary.

So lets write one. We'll write an authentication middleware, so that you can keep your greeting from being seen by just anyone.

Let's use HTTP authentication, which also can mystify people a bit. HTTP authentication is fairly simple:

• When authentication is requires, we give a 401 Authentication Required status with a WWW-Authenticate: Basic realm="This Realm" header
• The client then sends back a header Authorization: Basic encoded_info
• The "encoded_info" is a base-64 encoded version of username:password

So how does this work? Well, we're writing "middleware", which means we'll typically pass the request on to another application. We could change the request, or change the response, but in this case sometimes we won't pass the request on (like, when we need to give that 401 response).

To give an example of a really really simple middleware, here's one that capitalizes the response:

class Capitalizer(object):

    # We generally pass in the application to be wrapped to
    # the middleware constructor:
    def __init__(self, wrap_app):
        self.wrap_app = wrap_app

    def __call__(self, environ, start_response):
        # We call the application we are wrapping with the
        # same arguments we get...
        response_iter = self.wrap_app(environ, start_response)
        # then change the response...
        response_string = ''.join(response_iter)
        return [response_string.upper()]

Techically this isn't quite right, because there there's two ways to return the response body, but we're skimming bits. paste.wsgilib.intercept_output is a somewhat more thorough implementation of this.

So here's some code that does something useful, authentication:

class AuthMiddleware(object):

    def __init__(self, wrap_app):
        self.wrap_app = wrap_app

    def __call__(self, environ, start_response):
        if not self.authorized(environ.get('HTTP_AUTHORIZATION')):
            # Essentially self.auth_required is a WSGI application
            # that only knows how to respond with 401...
            return self.auth_required(environ, start_response)
        # But if everything is okay, then pass everything through
        # to the application we are wrapping...
        return self.wrap_app(environ, start_response)

    def authorized(self, auth_header):
        if not auth_header:
            # If they didn't give a header, they better login...
            return False
        # .split(None, 1) means split in two parts on whitespace:
        auth_type, encoded_info = auth_header.split(None, 1)
        assert auth_type.lower() == 'basic'
        unencoded_info = encoded_info.decode('base64')
        username, password = unencoded_info.split(':', 1)
        return self.check_password(username, password)

    def check_password(self, username, password):
        # Not very high security authentication...
        return username == password

    def auth_required(self, environ, start_response):
        start_response('401 Authentication Required',
            [('Content-type', 'text/html'),
             ('WWW-Authenticate', 'Basic realm="this realm"')])
        return ["""
        <html>
         <head><title>Authentication Required</title></head>
         <body>
          <h1>Authentication Required</h1>
          If you can't get in, then stay out.
         </body>
        </html>"""]

So, how do we use this?

app = ObjectPublisher(Root())
wrapped_app = AuthMiddleware(app)

if __name__ == '__main__':
    from paste import httpserver
    httpserver.serve(wrapped_app, host='127.0.0.1', port='8080')

Now you have middleware! Hurrah!

app = ObjectPublisher(Root())
wrapped_app = AuthMiddleware(app)
from paste.exceptions.errormiddleware import ErrorMiddleware
exc_wrapped_app = ErrorMiddleware(wrapped_app)

app = ObjectPublisher(Root())
wrapped_app = AuthMiddleware(app)
from paste.evalexception import EvalException
exc_wrapped_app = EvalException(wrapped_app)

Configuration

Now that you've created your framework and application (I'm sure it's much nicer than the one I've given so far) you might find the manually plugging together of some of these pieces a little crude. Well, if you don't, anyone else who uses your app and wants to install it in a different location or configure it differently won't be as happy.

So, we want to separate the application setup from the application configuration.

What's Next?

Stay tuned, I'll be talking about configuration (using Paste Deploy) later, and I hope to give a short introduction to packaging and plugins as well. When that happens I'll note it on my blog.