A Do-It-Yourself Framework

author:

Ian Bicking <ianb@colorstudy.com>

revision:

$Rev$

date:

$LastChangedDate$

This tutorial has been translated into Portuguese.

A newer version of this article is available using WebOb.

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.

An Interactive App

That last app wasn’t very interesting. Let’s at least make it interactive. To do that we’ll give a form, and then parse the form fields:

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>']

The parse_formvars function just takes the WSGI environment and turns that into a MultiDict which may contain FieldStorage instances.

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.

Object Publishing

In a typical Python object publisher you translate / to .. So /articles/view?id=5 turns into root.articles.view(id=5). We have to start with some root object, of course, which we’ll pass in…

class ObjectPublisher:

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

    def __call__(self, environ, start_response):
        ...

app = ObjectPublisher(my_root_object)

We override __call__ to make instances of ObjectPublisher callable objects, just like a function, and just like WSGI applications. Now all we have to do is translate that environ into the thing we are publishing, then call that thing, then turn the response into what WSGI wants.

The Path

WSGI puts the requested path into two variables: SCRIPT_NAME and PATH_INFO. SCRIPT_NAME is everything that was used up getting here. PATH_INFO is everything left over – it’s the part the framework should be using to find the object. If you put the two back together, you get the full path used to get to where we are right now; this is very useful for generating correct URLs, and we’ll make sure we preserve this.

So here’s how we might implement __call__:

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)

And that’s it, we’ve got a framework.

Taking It For a Ride

Now, let’s write a little application. Put that ObjectPublisher class into a module objectpub:

from objectpub import ObjectPublisher

class Root:

    # 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')

Alright, done! Oh, wait. There’s still some big missing features, like how do you set headers? And instead of giving 404 Not Found responses in some places, you’ll just get an attribute error. We’ll fix those up in a later installment…

Give Me More!

You’ll notice some things are missing here. Most specifically, there’s no way to set the output headers, and the information on the request is a little slim.

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

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

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

Now I’ll teach you a little trick. We don’t want to change the signature of the methods. But we can’t put the request and response objects in normal global variables, because we want to be thread-friendly, and all threads see the same global variables (even if they are processing different requests).

But Python 2.4 introduced a concept of “thread-local values”. That’s a value that just this one thread can see. This is in the threading.local object. When you create an instance of local any attributes you set on that object can only be seen by the thread you set them in. So we’ll attach the request and response objects here.

So, let’s remind ourselves of what the __call__ function looked like:

class ObjectPublisher:
    ...

    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]

Lets’s update that:

import threading
webinfo = threading.local()

class ObjectPublisher:
    ...

    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]

Now in our method we might do:

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

If we were being fancier we would do things like handle cookies in these objects. But we aren’t going to do that now. You have a framework, be happy!

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:

    # 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.

Note

This, like a lot of parts of this (now fairly old) tutorial is better, more thorough, and easier using WebOb. This particular example looks like:

from webob import Request

class Capitalizer:
    def __init__(self, app):
        self.app = app
    def __call__(self, environ, start_response):
        req = Request(environ)
        resp = req.get_response(self.app)
        resp.body = resp.body.upper()
        return resp(environ, start_response)

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

class AuthMiddleware:

    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>"""]

Note

Again, here’s the same thing with WebOb:

from webob import Request, Response

class AuthMiddleware:
    def __init__(self, app):
        self.app = app
    def __call__(self, environ, start_response):
        req = Request(environ)
        if not self.authorized(req.headers['authorization']):
            resp = self.auth_required(req)
        else:
            resp = self.app
        return resp(environ, start_response)
    def authorized(self, header):
        if not header:
            return False
        auth_type, encoded = header.split(None, 1)
        if not auth_type.lower() == 'basic':
            return False
        username, password = encoded.decode('base64').split(':', 1)
        return self.check_password(username, password)
 def check_password(self, username, password):
     return username == password
 def auth_required(self, req):
     return Response(status=401, headers={'WWW-Authenticate': 'Basic realm="this realm"'},
                     body="""\
     <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!

Give Me More Middleware!

It’s even easier to use other people’s middleware than to make your own, because then you don’t have to program. If you’ve been following along, you’ve probably encountered a few exceptions, and have to look at the console to see the exception reports. Let’s make that a little easier, and show the exceptions in the browser…

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

Easy! But let’s make it more fancy…

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

So go make an error now. And hit the little +’s. And type stuff in to the boxes.

Conclusion

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 keep writing it (many people have so far), but even if you don’t you should be able to recognize these components in other frameworks now, and you’ll have a better understanding how they probably work under the covers.

Also check out the version of this tutorial written using WebOb. That tutorial includes things like testing and pattern-matching dispatch (instead of object publishing).