1   Introduction

If you haven't done so already read the installation instructions and getting started guide.

In this tutorial we are going to create a working wiki from scratch using Pylons 0.9.6 and SQLAlchemy. Our wiki will allow visitors to add, edit or delete formatted wiki pages.

2   Starting at the End

Pylons is designed to be easy for everyone, not just developers, so lets start by downloading and installing the finished QuickWiki in exactly the way end users of QuickWiki might do. Once we have explored its features we will set about writing it from scratch.

After you have installed Easy Install run these commands to install QuickWiki and create a config file:

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$ easy_install QuickWiki==0.1.5
$ paster make-config QuickWiki test.ini

Next edit the configuration file by specifying the sqlalchemy.default.url variable in [app:main] section so that the data source name points to the database you wish to use.

Finally create the database tables and serve the finished application:

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$ paster setup-app test.ini
$ paster serve test.ini

That's it! Now you can visit http://127.0.0.1:5000 and experiment with the finished Wiki. Note that in the title list screen you can drag page titles to the trash area to delete them via AJAX calls.

When you've finished, stop the server with CTRL+C because we will start developing our own version.

If you are interested in looking at the latest version of the QuickWiki source code it can be browsed online at http://www.knowledgetap.com/hg/QuickWiki or can be checked out using Mercurial:

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$ hg clone http://www.knowledgetap.com/hg/QuickWiki

3   Developing QuickWiki

If you skipped the "Starting at the End" section you will need to assure that you have Pylons installed. See the installation instructions, or in a nutshell:

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$ easy_install -U Pylons>=0.9.6

Then create your project:

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$ paster create -t pylons QuickWiki

Now lets start the server and see what we have:

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$ cd QuickWiki
$ paster serve --reload development.ini

Open a new console and cd QuickWiki/quickwiki. Visit http://127.0.0.1:5000 where you will see the introduction page. Delete the file public/index.html because we want to see the front page of the wiki instead of this welcome page. If you now refresh the page, the Pylons built-in error document support will kick in and display an Error 404 page to tell you the file could not be found. We'll setup a controller to handle this location later.

4   The Model

Pylons uses a Model View Controller architecture; we'll start by creating the model. We could use any system we like for the model including SQLObject or SQLAlchemy. SQLAlchemy is the default for current versions of Pylons, and we'll use it for QuickWiki.

The most basic way of using SQLAlchemy is with explicit sessions where you create Session objects as needed. Pylons applications typically employ a slightly more sophisticated setup using SQLAlchemy 0.4's "contextual," thread-local sessions, via scoped_session. With this configuration, the application can use a single Session instance per web request, without the need to pass it around explicitly. Instantiating a new Session will actually find an existing one in the current thread if available. This is also covered in the document Using SQLAlchemy with Pylons, and you can learn further details in the SQLAlchemy documentation on the Session.

Now replace the contents of your model/__init__.py file so that it looks like this:

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from pylons import config
from sqlalchemy import Column, MetaData, Table, types
from sqlalchemy.orm import mapper
from sqlalchemy.orm import scoped_session, sessionmaker

Session = scoped_session(sessionmaker(autoflush=True, transactional=True,
                                      bind=config['pylons.g'].sa_engine))

metadata = MetaData()

pages_table = Table('pages', metadata,
    Column('title', types.Unicode(40), primary_key=True),
    Column('content', types.Unicode(), default='')
)

The first line imports Pylons' config object so we can bind our database Session to an engine -- more on that in a bit. The second line imports some useful SQLAlchemy objects such as the Table and Column classes. The third imports the mapper function which we use to map our table schemas to objects. The final import statement provides two functions for setting up the session and adding the contextual functionality.

After the imports we setup our metadata object which is used when defining and managing tables. We then define a table called pages which has two columns, title (the primary key) and content.

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pages_table = Table('pages', metadata, autoload=True)

A core philosophy of SQLAlchemy is that tables and domain classes are different beasts. So next, we'll create the Python class that will represent the pages of our wiki and map these domain objects to rows in the pages table using a mapper. In a more complex application, you could break out model classes into separate .py files in your model directory, but for sake of simplicity in this case, we'll just stick to __init__.py.

Add this to the bottom of model/__init__.py:

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class Page(object):
    def __str__(self):
        return self.title

mapper(Page, pages_table)

For those familiar with SQLAlchemy 0.3, scoped_session replaces the sessioncontext extension, and Session.mapper could then be used here in place of mapper to get behavior similar to what used to be achieved with assign_mapper. This is considered an advanced topic, and you should consult SQLAlchemy's documentation if you wish to learn how it works.

Looking ahead, our wiki will need some formatting so we will need to turn the content field into HTML. Any WikiWords (which are words made by joining together two or more lowercase words with the first letter capitalized) will also need to be converted into hyperlinks.

It would be nice if we could add a method to our Page object to retrieve the formatted HTML with the WikiWords already converted to hyperlinks. Add the following at the top of the model/__init__.py file:

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import re
import sets

from docutils.core import publish_parts

import quickwiki.lib.helpers as h

wikiwords = re.compile(r"\b([A-Z]\w+[A-Z]+\w+)", re.UNICODE)

and then add a get_wiki_content() method to the Page object so it looks like this:

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class Page(object):
    content = None

    def __str__(self):
        return self.title

    def get_wiki_content(self):
        content = publish_parts(self.content, writer_name="html")["html_body"]
        titles = sets.Set(wikiwords.findall(content))
        for title in titles:
            title_url = h.url_for(controller='page', action='index',
                                  title=title)
            content = content.replace(title, h.link_to(title, title_url))
        return content

This code deserves a bit of explaining. The content = None line is so that the content attribute is initialized to None when a new Page object is created. The Page object represents a row in the pages table so self.content will be the value of the content field. The Set object provides us with only unique WikiWord names, so we don't try replacing them more than once (a "wikiword" is of course defined by the regular expression set globally). h.link_to() and h.url_for() are standard Pylons helpers which create links to specific controller actions. In this case we have decided that all WikiWords should link to the index action of the page controller which we will create later.

One final change, since we have used docutils and SQLAlchemy, both third party packages, we need to edit our setup.py file so that anyone installing QuickWiki with Easy Install will automatically also have these dependencies installed for them too. Edit your setup.py in your project root directory so that the install_requires line looks like this:

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install_requires=["Pylons>=0.9.6", "docutils==0.4", "SQLAlchemy>=0.4.1"],

While we are we are making changes to setup.py we might want to complete some of the other sections too. Set the version number to 0.1.5 and add a description and URL which will be used on the Python Cheeseshop when we release it:

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version="0.1.5",
description="QuickWiki - Pylons 0.9.6 Tutorial application",
url="http://wiki.pylonshq.com/display/pylonsdocs/QuickWiki+Tutorial",

We might also want to make a full release rather than a development release in which case we would remove the following lines from setup.cfg:

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[egg_info]
tag_build = dev
tag_svn_revision = true

To test the automatic installation of the dependencies, run the following command which will also install docutils and SQLAlchemy if you don't already have them:

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$ python setup.py develop

5   Configuration and Setup

Now lets make the changes necessary to enable QuickWiki to be set up by an end user. First, open environment.py from the config directory of your project. After from pylons import config, add the following import:

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from sqlalchemy import engine_from_config

Then, add this line at the end of the load_environment function:

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config['pylons.g'].sa_engine = engine_from_config(config,
                                                  'sqlalchemy.default.')

This creates an engine for each instance of your application, which manages connections and is the base level at which SQLAlchemy communicates with the database. The engine is added to Pylons' config object, where you earlier saw it accessed in the base parameter for setting up SQLAlchemy's Session.

Now edit websetup.py, used by the paster setup-app command, to look like this:

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"""Setup the QuickWiki application"""
import logging

from paste.deploy import appconfig
from pylons import config

from quickwiki.config.environment import load_environment

log = logging.getLogger(__name__)

def setup_config(command, filename, section, vars):
    """Place any commands to setup quickwiki here"""
    conf = appconfig('config:' + filename)
    load_environment(conf.global_conf, conf.local_conf)

    # Populate the DB on 'paster setup-app'
    import quickwiki.model as model

    log.info("Setting up database connectivity...")
    engine = config['pylons.g'].sa_engine
    log.info("Creating tables...")
    model.metadata.create_all(bind=engine)
    log.info("Successfully set up.")

    log.info("Adding front page data...")
    page = model.Page()
    page.title = 'FrontPage'
    page.content = 'Welcome to the QuickWiki front page.'
    model.Session.save(page)
    model.Session.commit()
    log.info("Successfully set up.")

You can see that environment.py's load_environment function is called, so our engine is ready and we can import the model. A SQLAlchemy MetaData object--which provides some utility methods for operating on database schema--usually needs to be connected to an engine, so the line model.metadata.create_all(bind=engine) uses the engine we've set up and, well, creates the table(s) we've defined. After the tables are created the other lines add some data for the simple front page to our wiki. Because we specified transactional=True when creating our Session, operations will be wrapped in a transaction and committed atomically (unless your DB doesn't support transactions, like MySQL's default MyISAM tables -- but that's beyond the scope of this tutorial).

To test this functionality run you first need to install your QuickWiki if you haven't already done so in order for paster to find the version we are developing instead of the version we installed at the very start:

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$ python setup.py develop

Specify your database URI in development.ini so that the [app:main] section contains something like this, customized as needed for your database:

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[app:main]
use = egg:QuickWiki
...
# Specify the database for SQLAlchemy to use.
# %(here) may include a ':' character on Windows environments; this can
# invalidate the URI when specifying a SQLite db via path name
sqlalchemy.default.url = sqlite:///%(here)s/quickwiki.db

If you want to see the SQL being generated, you can have SQLAlchemy echo it to the console by adding this line:

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sqlalchemy.default.echo = true

You can now run the paster setup-app command to setup your tables in the same way an end user would, remembering to drop and recreate the database if the version tested earlier has already created the tables:

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$ paster setup-app development.ini

At this stage you will need to ensure you have the appropriate Python database drivers for the database you chose, otherwise you might find SQLAlchemy complains it can't get the DBAPI module for the dialect it needs.

You should also edit QuickWiki.egg-info/paste_deploy_config.ini_tmpl so that when users run paster make-config the configuration file that is produced for them will already have a section telling them to enter their own database URI as we did when we installed the finished QuickWiki at the start of the tutorial. Add these lines in the [app:main] section:

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# Specify the database for SQLAlchemy to use.
# %(here) may include a ':' character on Windows environments; this can
# invalidate the URI when specifying a SQLite db via path name
#sqlalchemy.default.url = sqlite:///%(here)s/quickwiki.db
#sqlalchemy.default.echo = true

6   Templates

We will make use of a feature of the Mako templating language called inheritance for our project. Add the main page template in templates/base.mako:

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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
  "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html>
  <head>
    <title>QuickWiki</title>
    ${h.stylesheet_link_tag('/quick.css')}
    ${h.javascript_include_tag('/javascripts/effects.js', builtins=True)}
  </head>
  <body>
    <div class="content">
      ${next.body()}\
      <p class="footer">
        Return to the
        ${h.link_to('FrontPage', h.url_for(action="index", title="FrontPage"))}
        | ${h.link_to('Edit ' + c.title, h.url_for(title=c.title, action='edit'))}
      </p>
    </div>
  </body>
</html>

All our other templates will be automatically inserted into the ${next.body()} line and the whole page will be returned when we call the render() global from our controller so that we can easily apply a consistent theme to all our templates.

If you are interested in learning some of the features of Mako templates have a look at the comprehensive Mako Documentation. For now we just need to understand that next.body() is replaced with the child template and that anything within ${...} brackets is executed and replaced with the result.

This base.mako also makes use of various helper functions attached to the h object. These are described in the WebHelpers documentation. You can add more helpers to the h object by adding them to lib/helpers.py although for this project we don't need to do so.

7   Routing

Before we can add the actions we want to be able to route the requests to them correctly. Edit config/routing.py and adjust the 'Custom Routes' section to look like this:

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map.connect(':controller/:action/:title', controller='page',
            action='index', title='FrontPage')
map.connect(':title', controller='page', action='index', title='FrontPage')
map.connect('*url', controller='template', action='view')

Note that the default route has been replaced. This tells Pylons to route the root URL / to the index() method of the PageController class in page.py and specify the title argument as FrontPage. It also says that any URL of the form /SomePage should be routed to the same method but the title argument will contain the value of the first part of the URL, in this case SomePage. Any other URLs which can't be matched by these maps are routed to the template controller as usual where they will result in a 404 error page being displayed.

One of the main benefits of using the Routes system is that you can also create URLs automatically simply by specifying the routing arguments. For example if I want the URL for the page FrontPage I can create it with this code:

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h.url_for(title='FrontPage')

Although the URL would be fairly simple to create manually, with complicated URLs this approach is much quicker. It also has the significant advantage that if you ever deploy your Pylons application at a URL other than /, all the URLs will be automatically adjusted for the new path without you needing to make any manual modifications. This flexibility is a real advantage.

Full information on the powerful things you can do to route requests to controllers and actions can be found in the Routes manual.

8   Controllers

Quick Recap: We've setup the model, configured the application, added the routes and setup the base template in base.mako, now we need to write the application logic and we do this with controllers. In your project's root directory add a controller called page to your project with this command:

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$ paster controller page

If you are using Subversion, this will automatically be detected and the new controller and tests will be automatically added to your subversion repository.

We are going to need the following actions:

index(self, title)

displays a page based on the title

edit(self, title)

displays a from for editing the page title

save(self, title)

save the page title and show it with a saved message

list(self)

gives a list of all pages

delete(self)

deletes a page based on an AJAX drag and drop call

Let's get cracking! We just need to make one quick preparation first: edit the BaseController class that your new page controller subclasses, so that we get a clean Session each time one of your controllers is called. Open lib/base.py and edit the __call__ method like this:

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from quickwiki.model import Session

class BaseController(WSGIController):

    def __call__(self, environ, start_response):
        """Invoke the Controller"""
        # WSGIController.__call__ dispatches to the Controller method the
        # request is routed to. This routing information is available in
        # environ['pylons.routes_dict']
        try:
            return WSGIController.__call__(self, environ, start_response)
        finally:
            Session.remove()

This is critical for avoiding unexpected and hard-to-debug behavior resulting from old session data between requests.

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from quickwiki.model import Page

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def index(self, title):
    page_q = Session.query(Page)
    page = page_q.filter_by(title=title).first()
    if page:
        c.content = page.get_wiki_content()
        return render('/page.mako')
    elif model.wikiwords.match(title):
        return render('/new_page.mako')
    abort(404)

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<%inherit file="base.mako"/>

<h1 class="main">${c.title}</h1>
${c.content}

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<%inherit file="base.mako"/>

<h1 class="main">${c.title}</h1>
<p>This page doesn't exist yet.
  <a href="${h.url_for(action='edit', title=c.title)}">Create the page</a>.
</p>

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$ paster serve --reload development.ini

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body {
    background-color: #888;
    margin: 25px;
}
div.content{
    margin: 0;
    margin-bottom: 10px;
    background-color: #d3e0ea;
    border: 5px solid #333;
    padding: 5px 25px 25px 25px;
}
h1.main{
    width: 100%;
    border-bottom: 1px solid #000;
}
p.footer{
    width: 100%;
    padding-top: 3px;
    border-top: 1px solid #000;
}

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def edit(self, title):
    page_q = Session.query(Page)
    page = page_q.filter_by(title=title).first()
    if page:
        c.content = page.content
    return render('/edit.mako')

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<%inherit file="base.mako"/>

<h1 class="main">Editing ${c.title}</h1>

${h.start_form(h.url_for(action='save', title=c.title), method="post")}
  ${h.text_area(name='content', rows=7, cols=40, content=c.content)} <br />
  ${h.submit(value="Save changes", name='commit')}
${h.end_form()}

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def save(self, title):
    page_q = Session.query(Page)
    page = page_q.filter_by(title=title).first()
    if not page:
        page = model.Page()
        page.title = title
    page.content = request.POST.get('content','')
    c.title = page.title
    c.content = page.get_wiki_content()
    c.message = 'Successfully saved'
    Session.save_or_update(page)
    Session.commit()
    return render('/page.mako')

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% if c.message:
<p><div id="message">${c.message}</div></p>
% endif

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div#message{
    color: orangered;
}

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def list(self):
    c.titles = [page.title for page in Session.query(Page).all()]
    return render('/list.mako')

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<%inherit file="base.mako"/>

<h1 class="main">Title List</h1>

<ul id="titles">
% for title in c.titles:
<li>
    ${title}&nbsp;[${h.link_to('visit', h.url_for(title=title, action="index"))}]
</li>
% endfor
</ul>

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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
  "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html>
  <head>
    <title>QuickWiki</title>
    ${h.stylesheet_link_tag('/quick.css')}
    ${h.javascript_include_tag('/javascripts/effects.js', builtins=True)}
  </head>
  <body>
    <div class="content">
      ${next.body()}\
      <p class="footer">
      ${footer(request.environ['pylons.routes_dict']['action'])}\
      </p>
    </div>
  </body>
</html>

## Don't show links that are redundant for particular pages
<%def name="footer(action)">\
  Return to the ${h.link_to('FrontPage', h.url_for(action="index", title="FrontPage"))}
  % if action == "list":
    <% return '' %>
  % endif
  % if action != "edit":
    | ${h.link_to('Edit ' + c.title, h.url_for(title=c.title, action='edit'))}
  % endif
  | ${h.link_to('Title List', h.url_for(action='list', title=None))}
</%def>

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${h.javascript_include_tag('/javascripts/effects.js', builtins=True)}

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<script src="/javascripts/prototype.js" type="text/javascript"></script>
<script src="/javascripts/scriptaculous.js" type="text/javascript"></script>
<script src="/javascripts/effects.js" type="text/javascript"></script>

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javascripts_app = StaticJavascripts()
app = Cascade([static_app, javascripts_app, app])

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<%inherit file="base.mako"/>

<h1 class="main">Title List</h1>

<ul id="titles">
  <%include file="list-titles.mako"/>
</ul>

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% for title in c.titles:
<li>
  <span id="${unicode(title)}">${title}</span>
  &nbsp;[${h.link_to('visit', h.url_for(title=title, action="index"))}]
  ${h.draggable_element(unicode(title), revert=True)}
</li>
% endfor

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<div id="trash">
    Delete a page by dragging its title here
</div>
${h.drop_receiving_element("trash", update="titles", url=h.url_for(action="delete"))}

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div#trash{
    float: right;
    margin: 0px 20px 20px 20px;
    background: #eee;
    border: 2px