thirdparty/google_appengine/lib/webob/docs/comment-example.txt
author Sverre Rabbelier <srabbelier@gmail.com>
Sun, 04 Oct 2009 23:43:36 +0200
changeset 3012 51eeb0c4317e
parent 109 620f9b141567
permissions -rw-r--r--
Set new Melange version number to 0-5-20091004 in app.yaml.template.
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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Comment Example
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===============
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.. contents::
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Introduction
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------------
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This is an example of how to write WSGI middleware with WebOb.  The
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specific example adds a simple comment form to HTML web pages; any
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page served through the middleware that is HTML gets a comment form
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added to it, and shows any existing comments.
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Code
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----
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The finished code for this is available in
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`docs/comment-example-code/example.py
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<http://svn.pythonpaste.org/Paste/WebOb/trunk/docs/comment-example-code/example.py>`_
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-- you can run that file as a script to try it out.
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Instantiating Middleware
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------------------------
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Middleware of any complexity at all is usually best created as a
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class with its configuration as arguments to that class.
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Every middleware needs an application (``app``) that it wraps.  This
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middleware also needs a location to store the comments; we'll put them
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all in a single directory.
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.. code-block::
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    import os
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    class Commenter(object):
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        def __init__(self, app, storage_dir):
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            self.app = app
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            self.storage_dir = storage_dir
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            if not os.path.exists(storage_dir):
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                os.makedirs(storage_dir)
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When you use this middleware, you'll use it like:
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.. code-block::
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    app = ... make the application ...
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    app = Commenter(app, storage_dir='./comments')
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For our application we'll use a simple static file server that is
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included with `Paste <http://pythonpaste.org>`_ (use ``easy_install
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Paste`` to install this).  The setup is all at the bottom of
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``example.py``, and looks like this:
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.. code-block::
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    if __name__ == '__main__':
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        import optparse
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        parser = optparse.OptionParser(
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            usage='%prog --port=PORT BASE_DIRECTORY'
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            )
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        parser.add_option(
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            '-p', '--port',
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            default='8080',
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            dest='port',
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            type='int',
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            help='Port to serve on (default 8080)')
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        parser.add_option(
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            '--comment-data',
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            default='./comments',
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            dest='comment_data',
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            help='Place to put comment data into (default ./comments/)')
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        options, args = parser.parse_args()
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        if not args:
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            parser.error('You must give a BASE_DIRECTORY')
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        base_dir = args[0]
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        from paste.urlparser import StaticURLParser
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        app = StaticURLParser(base_dir)
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        app = Commenter(app, options.comment_data)
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        from wsgiref.simple_server import make_server
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        httpd = make_server('localhost', options.port, app)
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        print 'Serving on http://localhost:%s' % options.port
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        try:
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            httpd.serve_forever()
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        except KeyboardInterrupt:
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            print '^C'
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I won't explain it here, but basically it takes some options, creates
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an application that serves static files
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(``StaticURLParser(base_dir)``), wraps it with ``Commenter(app,
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options.comment_data)`` then serves that.
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The Middleware
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--------------
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While we've created the class structure for the middleware, it doesn't
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actually do anything.  Here's a kind of minimal version of the
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middleware (using WebOb):
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.. code-block::
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    from webob import Request
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    class Commenter(object):
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        def __init__(self, app, storage_dir):
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            self.app = app
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            self.storage_dir = storage_dir
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            if not os.path.exists(storage_dir):
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                os.makedirs(storage_dir)
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        def __call__(self, environ, start_response):
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            req = Request(environ)
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            resp = req.get_response(self.app)
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            return resp(environ, start_response)
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This doesn't modify the response it any way.  You could write it like
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this without WebOb:
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.. code-block::
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    class Commenter(object):
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        ...
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        def __call__(self, environ, start_response):
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            return self.app(environ, start_response)
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But it won't be as convenient later.  First, lets create a little bit
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of infrastructure for our middleware.  We need to save and load
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per-url data (the comments themselves).  We'll keep them in pickles,
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where each url has a pickle named after the url (but double-quoted, so
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``http://localhost:8080/index.html`` becomes
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``http%3A%2F%2Flocalhost%3A8080%2Findex.html``).
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.. code-block::
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    from cPickle import load, dump
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    class Commenter(object):
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        ...
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        def get_data(self, url):
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            filename = self.url_filename(url)
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            if not os.path.exists(filename):
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                return []
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            else:
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                f = open(filename, 'rb')
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                data = load(f)
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                f.close()
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                return data
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        def save_data(self, url, data):
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            filename = self.url_filename(url)
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            f = open(filename, 'wb')
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            dump(data, f)
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            f.close()
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        def url_filename(self, url):
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            # Double-quoting makes the filename safe
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            return os.path.join(self.storage_dir, urllib.quote(url, ''))
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You can get the full request URL with ``req.url``, so to get the
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comment data with these methods you do ``data =
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self.get_data(req.url)``.
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Now we'll update the ``__call__`` method to filter *some* responses,
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and get the comment data for those.  We don't want to change responses
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that were error responses (anything but ``200``), nor do we want to
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filter responses that aren't HTML.  So we get:
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.. code-block::
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    class Commenter(object):
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        ...
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        def __call__(self, environ, start_response):
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            req = Request(environ)
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            resp = req.get_response(self.app)
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            if resp.content_type != 'text/html' or resp.status_int != 200:
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                return resp(environ, start_response)
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            data = self.get_data(req.url)
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            ... do stuff with data, update resp ...
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            return resp(environ, start_response)
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So far we're punting on actually adding the comments to the page.  We
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also haven't defined what ``data`` will hold.  Let's say it's a list
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of dictionaries, where each dictionary looks like ``{'name': 'John
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Doe', 'homepage': 'http://blog.johndoe.com', 'comments': 'Great
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site!'}``.
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We'll also need a simple method to add stuff to the page.  We'll use a
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regular expression to find the end of the page and put text in:
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.. code-block::
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    import re
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    class Commenter(object):
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        ...
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        _end_body_re = re.compile(r'</body.*?>', re.I|re.S)
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        def add_to_end(self, html, extra_html):
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            """
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            Adds extra_html to the end of the html page (before </body>)
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            """
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            match = self._end_body_re.search(html)
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            if not match:
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                return html + extra_html
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            else:
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                return html[:match.start()] + extra_html + html[match.start():]
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And then we'll use it like:
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.. code-block::
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    data = self.get_data(req.url)
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    body = resp.body
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    body = self.add_to_end(body, self.format_comments(data))
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    resp.body = body
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    return resp(environ, start_response)
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We get the body, update it, and put it back in the response.  This
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also updates ``Content-Length``.  Then we define:
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.. code-block::
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    from webob import html_escape
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    class Commenter(object):
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        ...
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        def format_comments(self, comments):
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            if not comments:
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                return ''
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            text = []
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            text.append('<hr>')
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            text.append('<h2><a name="comment-area"></a>Comments (%s):</h2>' % len(comments))
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            for comment in comments:
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                text.append('<h3><a href="%s">%s</a> at %s:</h3>' % (
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                    html_escape(comment['homepage']), html_escape(comment['name']), 
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                    time.strftime('%c', comment['time'])))
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                # Susceptible to XSS attacks!:
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                text.append(comment['comments'])
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            return ''.join(text)
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We put in a header (with an anchor we'll use later), and a section for
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each comment.  Note that ``html_escape`` is the same as ``cgi.escape``
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and just turns ``&`` into ``&amp;``, etc.  
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Because we put in some text without quoting it is susceptible to a
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`Cross-Site Scripting
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<http://en.wikipedia.org/wiki/Cross-site_scripting>`_ attack.  Fixing
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that is beyond the scope of this tutorial; you could quote it or clean
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it with something like `lxml.html.clean
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<http://codespeak.net/lxml/lxmlhtml.html#cleaning-up-html>`_.
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Accepting Comments
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------------------
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All of those pieces *display* comments, but still no one can actually
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make comments.  To handle this we'll take a little piece of the URL
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space for our own, everything under ``/.comments``, so when someone
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POSTs there it will add a comment.
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When the request comes in there are two parts to the path:
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``SCRIPT_NAME`` and ``PATH_INFO``.  Everything in ``SCRIPT_NAME`` has
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already been parsed, and everything in ``PATH_INFO`` has yet to be
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parsed.  That means that the URL *without* ``PATH_INFO`` is the path
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to the middleware; we can intercept anything else below
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``SCRIPT_NAME`` but nothing above it.  The name for the URL without
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``PATH_INFO`` is ``req.application_url``.  We have to capture it early
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to make sure it doesn't change (since the WSGI application we are
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wrapping may update ``SCRIPT_NAME`` and ``PATH_INFO``).
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So here's what this all looks like:
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.. code-block::
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    class Commenter(object):
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        ...
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        def __call__(self, environ, start_response):
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            req = Request(environ)
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            if req.path_info_peek() == '.comments':
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                return self.process_comment(req)(environ, start_response)
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            # This is the base path of *this* middleware:
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            base_url = req.application_url
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            resp = req.get_response(self.app)
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            if resp.content_type != 'text/html' or resp.status_int != 200:
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                # Not an HTML response, we don't want to
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                # do anything to it
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                return resp(environ, start_response)
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            # Make sure the content isn't gzipped:
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            resp.decode_content()
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            comments = self.get_data(req.url)
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            body = resp.body
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            body = self.add_to_end(body, self.format_comments(comments))
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            body = self.add_to_end(body, self.submit_form(base_url, req))
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            resp.body = body
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            return resp(environ, start_response)
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``base_url`` is the path where the middleware is located (if you run
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the example server, it will be ``http://localhost:PORT/``).  We use
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``req.path_info_peek()`` to look at the next segment of the URL --
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what comes after base_url.  If it is ``.comments`` then we handle it
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internally and don't pass the request on.
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We also put in a little guard, ``resp.decode_content()`` in case the
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application returns a gzipped response.
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Then we get the data, add the comments, add the *form* to make new
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comments, and return the result.
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submit_form
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~~~~~~~~~~~
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Here's what the form looks like:
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.. code-block::
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    class Commenter(object):
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        ...
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        def submit_form(self, base_path, req):
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            return '''<h2>Leave a comment:</h2>
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            <form action="%s/.comments" method="POST">
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             <input type="hidden" name="url" value="%s">
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             <table width="100%%">
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   329
              <tr><td>Name:</td>
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   330
                  <td><input type="text" name="name" style="width: 100%%"></td></tr>
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   331
              <tr><td>URL:</td>
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   332
                  <td><input type="text" name="homepage" style="width: 100%%"></td></tr>
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   333
             </table>
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   334
             Comments:<br>
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   335
             <textarea name="comments" rows=10 style="width: 100%%"></textarea><br>
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   336
             <input type="submit" value="Submit comment">
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   337
            </form>
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            ''' % (base_path, html_escape(req.url))
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Nothing too exciting.  It submits a form with the keys ``url`` (the
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URL being commented on), ``name``, ``homepage``, and ``comments``.
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process_comment
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~~~~~~~~~~~~~~~
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If you look at the method call, what we do is call the method then
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treat the result as a WSGI application:
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.. code-block::
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    return self.process_comment(req)(environ, start_response)
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   353
You could write this as:
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.. code-block::
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    response = self.process_comment(req)
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    return response(environ, start_response)
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   359
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A common pattern in WSGI middleware that *doesn't* use WebOb is to
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   361
just do:
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   362
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.. code-block::
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   364
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    return self.process_comment(environ, start_response)
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   366
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But the WebOb style makes it easier to modify the response if you want
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   368
to; modifying a traditional WSGI response/application output requires
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   369
changing your logic flow considerably.
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   370
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   371
Here's the actual processing code:
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   372
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.. code-block::
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   375
    from webob import exc
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    from webob import Response
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    class Commenter(object):
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        ...
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        def process_comment(self, req):
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   382
            try:
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   383
                url = req.params['url']
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parents:
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   384
                name = req.params['name']
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   385
                homepage = req.params['homepage']
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   386
                comments = req.params['comments']
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   387
            except KeyError, e:
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   388
                resp = exc.HTTPBadRequest('Missing parameter: %s' % e)
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   389
                return resp
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   390
            data = self.get_data(url)
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   391
            data.append(dict(
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   392
                name=name,
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parents:
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   393
                homepage=homepage,
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parents:
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   394
                comments=comments,
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   395
                time=time.gmtime()))
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parents:
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   396
            self.save_data(url, data)
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parents:
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   397
            resp = exc.HTTPSeeOther(location=url+'#comment-area')
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parents:
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   398
            return resp
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parents:
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   399
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   400
We either give a Bad Request response (if the form submission is
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parents:
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   401
somehow malformed), or a redirect back to the original page.
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parents:
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   402
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parents:
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   403
The classes in ``webob.exc`` (like ``HTTPBadRequest`` and
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parents:
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   404
``HTTPSeeOther``) are Response subclasses that can be used to quickly
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parents:
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   405
create responses for these non-200 cases where the response body
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parents:
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   406
usually doesn't matter much.
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parents:
diff changeset
   407
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parents:
diff changeset
   408
Conclusion
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parents:
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   409
----------
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parents:
diff changeset
   410
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parents:
diff changeset
   411
This shows how to make response modifying middleware, which is
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parents:
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   412
probably the most difficult kind of middleware to write with WSGI --
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parents:
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   413
modifying the request is quite simple in comparison, as you simply
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parents:
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   414
update ``environ``.