third_party/Paste/docs/do-it-yourself-framework.txt - external/trace-viewer - Git at Google

 A Do-It-Yourself Framework
 ++++++++++++++++++++++++++

 :author: Ian Bicking <ianb@colorstudy.com>
 :revision: $Rev$
 :date: $LastChangedDate$

 This tutorial has been translated `into Portuguese
 <http://montegasppa.blogspot.com/2007/06/um-framework-faa-voc-mesmo.html>`_.

 A newer version of this article is available `using WebOb
 <http://pythonpaste.org/webob/do-it-yourself.html>`_.

 .. contents::

 .. comments:

    Explain SCRIPT_NAME/PATH_INFO better

 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
 <http://www.python.org/peps/pep-0333.html>`_ 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
 <http://www.python.org/peps/pep-0333.html#the-server-gateway-side>`_
 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
 calls the `cgi <http://python.org/doc/current/lib/module-cgi.html>`_
 module (the ``FieldStorage`` class) and turns that into a MultiDict.

 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(object):

         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(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')

 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(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'})

 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 <http://docs.python.org/lib/module-threading.html>`_
 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(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]

 Lets's update that::

     import threading
     webinfo = threading.local()

     class ObjectPublisher(object):
         ...

         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
 <http://python.org/doc/current/lib/module-Cookie.html>`_ in these
 objects.  But we aren't going to do that now.  You have a framework,
 be happy!

 WSGI Middleware
 ===============

 `Middleware
 <http://www.python.org/peps/pep-0333.html#middleware-components-that-play-both-sides>`_
 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
 <http://pythonpaste.org/module-paste.wsgilib.html#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
    <http://pythonpaste.org/webob/>`_.  This particular example looks
    like::

        from webob import Request

        class Capitalizer(object):
            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(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>"""]

 .. note::

    Again, here's the same thing with WebOb::

        from webob import Request, Response

        class AuthMiddleware(object):
            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
 <http://pythonpaste.org/webob/do-it-yourself.html>`_.  That tutorial
 includes things like **testing** and **pattern-matching dispatch**
 (instead of object publishing).
	A Do-It-Yourself Framework
	++++++++++++++++++++++++++

	:author: Ian Bicking <ianb@colorstudy.com>
	:revision: $Rev$
	:date: $LastChangedDate$

	This tutorial has been translated `into Portuguese
	<http://montegasppa.blogspot.com/2007/06/um-framework-faa-voc-mesmo.html>`_.

	A newer version of this article is available `using WebOb
	<http://pythonpaste.org/webob/do-it-yourself.html>`_.

	.. contents::

	.. comments:

	Explain SCRIPT_NAME/PATH_INFO better

	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
	<http://www.python.org/peps/pep-0333.html>`_ 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
	<http://www.python.org/peps/pep-0333.html#the-server-gateway-side>`_
	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
	calls the `cgi <http://python.org/doc/current/lib/module-cgi.html>`_
	module (the ``FieldStorage`` class) and turns that into a MultiDict.

	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(object):

	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(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')

	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(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'})

	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 <http://docs.python.org/lib/module-threading.html>`_
	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(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]

	Lets's update that::

	import threading
	webinfo = threading.local()

	class ObjectPublisher(object):
	...

	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
	<http://python.org/doc/current/lib/module-Cookie.html>`_ in these
	objects. But we aren't going to do that now. You have a framework,
	be happy!

	WSGI Middleware
	===============

	`Middleware
	<http://www.python.org/peps/pep-0333.html#middleware-components-that-play-both-sides>`_
	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
	<http://pythonpaste.org/module-paste.wsgilib.html#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
	<http://pythonpaste.org/webob/>`_. This particular example looks
	like::

	from webob import Request

	class Capitalizer(object):
	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(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>"""]

	.. note::

	Again, here's the same thing with WebOb::

	from webob import Request, Response

	class AuthMiddleware(object):
	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
	<http://pythonpaste.org/webob/do-it-yourself.html>`_. That tutorial
	includes things like testing and pattern-matching dispatch
	(instead of object publishing).