wiki-example.txt

Wiki Example
============

:author: Ian Bicking <ianb@colorstudy.com>

.. contents::

Introduction
------------

This is an example of how to write a WSGI application using WebOb.
WebOb isn't itself intended to write applications -- it is not a web
framework on its own -- but it is *possible* to write applications
using just WebOb.

The `file serving example <file-example.html>`_ is a better example of
advanced HTTP usage.  The `comment middleware example
<comment-example.html>`_ is a better example of using middleware.
This example provides some completeness by showing an
application-focused end point.

This example implements a very simple wiki.

Code
----

The finished code for this is available in
`docs/wiki-example-code/example.py
<https://github.com/Pylons/webob/tree/master/docs/wiki-example-code/example.py>`_
-- you can run that file as a script to try it out.

Creating an Application
-----------------------

A common pattern for creating small WSGI applications is to have a
class which is instantiated with the configuration.  For our
application we'll be storing the pages under a directory.

.. code-block:: python

    class WikiApp(object):

        def __init__(self, storage_dir):
            self.storage_dir = os.path.abspath(os.path.normpath(storage_dir))

WSGI applications are callables like ``wsgi_app(environ,
start_response)``.  *Instances* of `WikiApp` are WSGI applications, so
we'll implement a ``__call__`` method:

.. code-block:: python

    class WikiApp(object):
        ...
        def __call__(self, environ, start_response):
            # what we'll fill in

To make the script runnable we'll create a simple command-line
interface:

.. code-block:: python

    if __name__ == '__main__':
        import optparse
        parser = optparse.OptionParser(
            usage='%prog --port=PORT'
            )
        parser.add_option(
            '-p', '--port',
            default='8080',
            dest='port',
            type='int',
            help='Port to serve on (default 8080)')
        parser.add_option(
            '--wiki-data',
            default='./wiki',
            dest='wiki_data',
            help='Place to put wiki data into (default ./wiki/)')
        options, args = parser.parse_args()
        print 'Writing wiki pages to %s' % options.wiki_data
        app = WikiApp(options.wiki_data)
        from wsgiref.simple_server import make_server
        httpd = make_server('localhost', options.port, app)
        print 'Serving on http://localhost:%s' % options.port
        try:
            httpd.serve_forever()
        except KeyboardInterrupt:
            print '^C'

There's not much to talk about in this code block.  The application is
instantiated and served with the built-in module
`wsgiref.simple_server
<http://www.python.org/doc/current/lib/module-wsgiref.simple_server.html>`_.

The WSGI Application
--------------------

Of course all the interesting stuff is in that ``__call__`` method.
WebOb lets you ignore some of the details of WSGI, like what
``start_response`` really is.  ``environ`` is a CGI-like dictionary,
but ``webob.Request`` gives an object interface to it.
``webob.Response`` represents a response, and is itself a WSGI
application.  Here's kind of the hello world of WSGI applications
using these objects:

.. code-block:: python

    from webob import Request, Response

    class WikiApp(object):
        ...

        def __call__(self, environ, start_response):
            req = Request(environ)
            resp = Response(
                'Hello %s!' % req.params.get('name', 'World'))
            return resp(environ, start_response)

``req.params.get('name', 'World')`` gets any query string parameter
(like ``?name=Bob``), or if it's a POST form request it will look for
a form parameter ``name``.  We instantiate the response with the body
of the response.  You could also give keyword arguments like
``content_type='text/plain'`` (``text/html`` is the default content
type and ``200 OK`` is the default status).

For the wiki application we'll support a couple different kinds of
screens, and we'll make our ``__call__`` method dispatch to different
methods depending on the request.  We'll support an ``action``
parameter like ``?action=edit``, and also dispatch on the method (GET,
POST, etc, in ``req.method``).  We'll pass in the request and expect a
response object back.

Also, WebOb has a series of exceptions in ``webob.exc``, like
``webob.exc.HTTPNotFound``, ``webob.exc.HTTPTemporaryRedirect``, etc.
We'll also let the method raise one of these exceptions and turn it
into a response.

One last thing we'll do in our ``__call__`` method is create our
``Page`` object, which represents a wiki page.

All this together makes:

.. code-block:: python

    from webob import Request, Response
    from webob import exc

    class WikiApp(object):
        ...

        def __call__(self, environ, start_response):
            req = Request(environ)
            action = req.params.get('action', 'view')
            # Here's where we get the Page domain object:
            page = self.get_page(req.path_info)
            try:
                try:
                    # The method name is action_{action_param}_{request_method}:
                    meth = getattr(self, 'action_%s_%s' % (action, req.method))
                except AttributeError:
                    # If the method wasn't found there must be
                    # something wrong with the request:
                    raise exc.HTTPBadRequest('No such action %r' % action)
                resp = meth(req, page)
            except exc.HTTPException, e:
                # The exception object itself is a WSGI application/response:
                resp = e
            return resp(environ, start_response)

The Domain Object
-----------------

The ``Page`` domain object isn't really related to the web, but it is
important to implementing this.  Each ``Page`` is just a file on the
filesystem.  Our ``get_page`` method figures out the filename given
the path (the path is in ``req.path_info``, which is all the path
after the base path).  The ``Page`` class handles getting and setting
the title and content.

Here's the method to figure out the filename:

.. code-block:: python

    import os

    class WikiApp(object):
        ...

        def get_page(self, path):
            path = path.lstrip('/')
            if not path:
                # The path was '/', the home page
                path = 'index'
            path = os.path.join(self.storage_dir)
            path = os.path.normpath(path)
            if path.endswith('/'):
                path += 'index'
            if not path.startswith(self.storage_dir):
                raise exc.HTTPBadRequest("Bad path")
            path += '.html'
            return Page(path)

Mostly this is just the kind of careful path construction you have to
do when mapping a URL to a filename.  While the server *may* normalize
the path (so that a path like ``/../../`` can't be requested), you can
never really be sure.  By using ``os.path.normpath`` we eliminate
these, and then we make absolutely sure that the resulting path is
under our ``self.storage_dir`` with ``if not
path.startswith(self.storage_dir): raise exc.HTTPBadRequest("Bad
path")``.

Here's the actual domain object:

.. code-block:: python

    class Page(object):
        def __init__(self, filename):
            self.filename = filename

        @property
        def exists(self):
            return os.path.exists(self.filename)

        @property
        def title(self):
            if not self.exists:
                # we need to guess the title
                basename = os.path.splitext(os.path.basename(self.filename))[0]
                basename = re.sub(r'[_-]', ' ', basename)
                return basename.capitalize()
            content = self.full_content
            match = re.search(r'<title>(.*?)</title>', content, re.I|re.S)
            return match.group(1)

        @property
        def full_content(self):
            f = open(self.filename, 'rb')
            try:
                return f.read()
            finally:
                f.close()

        @property
        def content(self):
            if not self.exists:
                return ''
            content = self.full_content
            match = re.search(r'<body[^>]*>(.*?)</body>', content, re.I|re.S)
            return match.group(1)

        @property
        def mtime(self):
            if not self.exists:
                return None
            else:
                return os.stat(self.filename).st_mtime

        def set(self, title, content):
            dir = os.path.dirname(self.filename)
            if not os.path.exists(dir):
                os.makedirs(dir)
            new_content = """<html><head><title>%s</title></head><body>%s</body></html>""" % (
                title, content)
            f = open(self.filename, 'wb')
            f.write(new_content)
            f.close()

Basically it provides a ``.title`` attribute, a ``.content``
attribute, the ``.mtime`` (last modified time), and the page can exist
or not (giving appropriate guesses for title and content when the page
does not exist).  It encodes these on the filesystem as a simple HTML
page that is parsed by some regular expressions.

None of this really applies much to the web or WebOb, so I'll leave it
to you to figure out the details of this.

URLs, PATH_INFO, and SCRIPT_NAME
--------------------------------

This is an aside for the tutorial, but an important concept.  In WSGI,
and accordingly with WebOb, the URL is split up into several pieces.
Some of these are obvious and some not.

An example::

  http://example.com:8080/wiki/article/12?version=10

There are several components here:

* req.scheme: ``http``
* req.host: ``example.com:8080``
* req.server_name: ``example.com``
* req.server_port: 8080
* req.script_name: ``/wiki``
* req.path_info: ``/article/12``
* req.query_string: ``version=10``

One non-obvious part is ``req.script_name`` and ``req.path_info``.
These correspond to the CGI environmental variables ``SCRIPT_NAME``
and ``PATH_INFO``.  ``req.script_name`` points to the *application*.
You might have several applications in your site at different paths:
one at ``/wiki``, one at ``/blog``, one at ``/``.  Each application
doesn't necessarily know about the others, but it has to construct its
URLs properly -- so any internal links to the wiki application should
start with ``/wiki``.

Just as there are pieces to the URL, there are several properties in
WebOb to construct URLs based on these:

* req.host_url: ``http://example.com:8080``
* req.application_url: ``http://example.com:8080/wiki``
* req.path_url: ``http://example.com:8080/wiki/article/12``
* req.path: ``/wiki/article/12``
* req.path_qs: ``/wiki/article/12?version=10``
* req.url: ``http://example.com:8080/wiki/article/12?version10``

You can also create URLs with
``req.relative_url('some/other/page')``.  In this example that would
resolve to ``http://example.com:8080/wiki/article/some/other/page``.
You can also create a relative URL to the application URL
(SCRIPT_NAME) like ``req.relative_url('some/other/page', True)`` which
would be ``http://example.com:8080/wiki/some/other/page``.

Back to the Application
-----------------------

We have a dispatching function with ``__call__`` and we have a domain
object with ``Page``, but we aren't actually doing anything.

The dispatching goes to ``action_ACTION_METHOD``, where ACTION
defaults to ``view``.  So a simple page view will be
``action_view_GET``.  Let's implement that:

.. code-block:: python

    class WikiApp(object):
        ...

        def action_view_GET(self, req, page):
            if not page.exists:
                return exc.HTTPTemporaryRedirect(
                    location=req.url + '?action=edit')
            text = self.view_template.substitute(
                page=page, req=req)
            resp = Response(text)
            resp.last_modified = page.mtime
            resp.conditional_response = True
            return resp

The first thing we do is redirect the user to the edit screen if the
page doesn't exist.  ``exc.HTTPTemporaryRedirect`` is a response that
gives a ``307 Temporary Redirect`` response with the given location.

Otherwise we fill in a template.  The template language we're going to
use in this example is `Tempita <http://pythonpaste.org/tempita/>`_, a
very simple template language with a similar interface to
`string.Template <http://python.org/doc/current/lib/node40.html>`_.

The template actually looks like this:

.. code-block:: python

    from tempita import HTMLTemplate

    VIEW_TEMPLATE = HTMLTemplate("""\
    <html>
     <head>
      <title>{{page.title}}</title>
     </head>
     <body>
    <h1>{{page.title}}</h1>

    <div>{{page.content|html}}</div>

    <hr>
    <a href="{{req.url}}?action=edit">Edit</a>
     </body>
    </html>
    """)

    class WikiApp(object):
        view_template = VIEW_TEMPLATE
        ...

As you can see it's a simple template using the title and the body,
and a link to the edit screen.  We copy the template object into a
class method (``view_template = VIEW_TEMPLATE``) so that potentially a
subclass could override these templates.

``tempita.HTMLTemplate`` is a template that does automatic HTML
escaping.  Our wiki will just be written in plain HTML, so we disable
escaping of the content with ``{{page.content|html}}``.

So let's look at the ``action_view_GET`` method again:

.. code-block:: python

        def action_view_GET(self, req, page):
            if not page.exists:
                return exc.HTTPTemporaryRedirect(
                    location=req.url + '?action=edit')
            text = self.view_template.substitute(
                page=page, req=req)
            resp = Response(text)
            resp.last_modified = page.mtime
            resp.conditional_response = True
            return resp

The template should be pretty obvious now.  We create a response with
``Response(text)``, which already has a default Content-Type of
``text/html``.

To allow conditional responses we set ``resp.last_modified``.  You can
set this attribute to a date, None (effectively removing the header),
a time tuple (like produced by ``time.localtime()``), or as in this
case to an integer timestamp.  If you get the value back it will
always be a `datetime
<http://python.org/doc/current/lib/datetime-datetime.html>`_ object
(or None).  With this header we can process requests with
If-Modified-Since headers, and return ``304 Not Modified`` if
appropriate.  It won't actually do that unless you set
``resp.conditional_response`` to True.

.. note::

    If you subclass ``webob.Response`` you can set the class attribute
    ``default_conditional_response = True`` and this setting will be
    on by default.  You can also set other defaults, like the
    ``default_charset`` (``"utf8"``), or ``default_content_type``
    (``"text/html"``).

The Edit Screen
---------------

The edit screen will be implemented in the method
``action_edit_GET``.  There's a template and a very simple method:

.. code-block:: python

    EDIT_TEMPLATE = HTMLTemplate("""\
    <html>
     <head>
      <title>Edit: {{page.title}}</title>
     </head>
     <body>
    {{if page.exists}}
    <h1>Edit: {{page.title}}</h1>
    {{else}}
    <h1>Create: {{page.title}}</h1>
    {{endif}}

    <form action="{{req.path_url}}" method="POST">
     <input type="hidden" name="mtime" value="{{page.mtime}}">
     Title: <input type="text" name="title" style="width: 70%" value="{{page.title}}"><br>
     Content: <input type="submit" value="Save">
     <a href="{{req.path_url}}">Cancel</a>
       <br>
     <textarea name="content" style="width: 100%; height: 75%" rows="40">{{page.content}}</textarea>
       <br>
     <input type="submit" value="Save">
     <a href="{{req.path_url}}">Cancel</a>
    </form>
    </body></html>
    """)

    class WikiApp(object):
        ...

        edit_template = EDIT_TEMPLATE

        def action_edit_GET(self, req, page):
            text = self.edit_template.substitute(
                page=page, req=req)
            return Response(text)

As you can see, all the action here is in the template.

In ``<form action="{{req.path_url}}" method="POST">`` we submit to
``req.path_url``; that's everything *but* ``?action=edit``.  So we are
POSTing right over the view page.  This has the nice side effect of
automatically invalidating any caches of the original page.  It also
is vaguely `RESTful
<http://en.wikipedia.org/wiki/Representational_State_Transfer>`_.

We save the last modified time in a hidden ``mtime`` field.  This way
we can detect concurrent updates.  If start editing the page who's
mtime is 100000, and someone else edits and saves a revision changing
the mtime to 100010, we can use this hidden field to detect that
conflict.  Actually resolving the conflict is a little tricky and
outside the scope of this particular tutorial, we'll just note the
conflict to the user in an error.

From there we just have a very straight-forward HTML form.  Note that
we don't quote the values because that is done automatically by
``HTMLTemplate``; if you are using something like ``string.Template``
or a templating language that doesn't do automatic quoting, you have
to be careful to quote all the field values.

We don't have any error conditions in our application, but if there
were error conditions we might have to re-display this form with the
input values the user already gave.  In that case we'd do something
like::

    <input type="text" name="title"
     value="{{req.params.get('title', page.title)}}">

This way we use the value in the request (``req.params`` is both the
query string parameters and any variables in a POST response), but if
there is no value (e.g., first request) then we use the page values.

Processing the Form
-------------------

The form submits to ``action_view_POST`` (``view`` is the default
action).  So we have to implement that method:

.. code-block:: python

    class WikiApp(object):
        ...

        def action_view_POST(self, req, page):
            submit_mtime = int(req.params.get('mtime') or '0') or None
            if page.mtime != submit_mtime:
                return exc.HTTPPreconditionFailed(
                    "The page has been updated since you started editing it")
            page.set(
                title=req.params['title'],
                content=req.params['content'])
            resp = exc.HTTPSeeOther(
                location=req.path_url)
            return resp

The first thing we do is check the mtime value.  It can be an empty
string (when there's no mtime, like when you are creating a page) or
an integer.  ``int(req.params.get('time') or '0') or None`` basically
makes sure we don't pass ``""`` to ``int()`` (which is an error) then
turns 0 into None (``0 or None`` will evaluate to None in Python --
``false_value or other_value`` in Python resolves to ``other_value``).
If it fails we just give a not-very-helpful error message, using ``412
Precondition Failed`` (typically preconditions are HTTP headers like
``If-Unmodified-Since``, but we can't really get the browser to send
requests like that, so we use the hidden field instead).

.. note::

    Error statuses in HTTP are often under-used because people think
    they need to either return an error (useful for machines) or an
    error message or interface (useful for humans).  In fact you can
    do both: you can give any human readable error message with your
    error response.

    One problem is that Internet Explorer will replace error messages
    with its own incredibly unhelpful error messages.  However, it
    will only do this if the error message is short.  If it's fairly
    large (4Kb is large enough) it will show the error message it was
    given.  You can load your error with a big HTML comment to
    accomplish this, like ``"<!-- %s -->" % ('x'*4000)``.

    You can change the status of any response with ``resp.status_int =
    412``, or you can change the body of an ``exc.HTTPSomething`` with
    ``resp.body = new_body``.  The primary advantage of using the
    classes in ``webob.exc`` is giving the response a clear name and a
    boilerplate error message.

After we check the mtime we get the form parameters from
``req.params`` and issue a redirect back to the original view page.
``303 See Other`` is a good response to give after accepting a POST
form submission, as it gets rid of the POST (no warning messages for the
user if they try to go back).

In this example we've used ``req.params`` for all the form values.  If
we wanted to be specific about where we get the values from, they
could come from ``req.GET`` (the query string, a misnomer since the
query string is present even in POST requests) or ``req.POST`` (a POST
form body).  While sometimes it's nice to distinguish between these
two locations, for the most part it doesn't matter.  If you want to
check the request method (e.g., make sure you can't change a page with
a GET request) there's no reason to do it by accessing these
method-specific getters.  It's better to just handle the method
specifically.  We do it here by including the request method in our
dispatcher (dispatching to ``action_view_GET`` or
``action_view_POST``).


Cookies
-------

One last little improvement we can do is show the user a message when
they update the page, so it's not quite so mysteriously just another
page view.

A simple way to do this is to set a cookie after the save, then
display it in the page view.  To set it on save, we add a little to
``action_view_POST``:

.. code-block:: python

    def action_view_POST(self, req, page):
        ...
        resp = exc.HTTPSeeOther(
            location=req.path_url)
        resp.set_cookie('message', 'Page updated')
        return resp

And then in ``action_view_GET``:

.. code-block:: python


    VIEW_TEMPLATE = HTMLTemplate("""\
    ...
    {{if message}}
    <div style="background-color: #99f">{{message}}</div>
    {{endif}}
    ...""")

    class WikiApp(object):
        ...

        def action_view_GET(self, req, page):
            ...
            if req.cookies.get('message'):
                message = req.cookies['message']
            else:
                message = None
            text = self.view_template.substitute(
                page=page, req=req, message=message)
            resp = Response(text)
            if message:
                resp.delete_cookie('message')
            else:
                resp.last_modified = page.mtime
                resp.conditional_response = True
            return resp

``req.cookies`` is just a dictionary, and we also delete the cookie if
it is present (so the message doesn't keep getting set).  The
conditional response stuff only applies when there isn't any
message, as messages are private.  Another alternative would be to
display the message with Javascript, like::

    <script type="text/javascript">
    function readCookie(name) {
        var nameEQ = name + "=";
        var ca = document.cookie.split(';');
        for (var i=0; i < ca.length; i++) {
            var c = ca[i];
            while (c.charAt(0) == ' ') c = c.substring(1,c.length);
            if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
        }
        return null;
    }

    function createCookie(name, value, days) {
        if (days) {
            var date = new Date();
            date.setTime(date.getTime()+(days*24*60*60*1000));
            var expires = "; expires="+date.toGMTString();
        } else {
            var expires = "";
        }
        document.cookie = name+"="+value+expires+"; path=/";
    }

    function eraseCookie(name) {
        createCookie(name, "", -1);
    }

    function showMessage() {
        var message = readCookie('message');
        if (message) {
            var el = document.getElementById('message');
            el.innerHTML = message;
            el.style.display = '';
            eraseCookie('message');
        }
    }
    </script>

Then put ``<div id="messaage" style="display: none"></div>`` in the
page somewhere.  This has the advantage of being very cacheable and
simple on the server side.

Conclusion
----------

We're done, hurrah!