=============
Grok tutorial
=============

.. raw:: html

   Also available as <a href="./tutorial.pdf">PDF</a>.

.. contents::

Welcome to the Grok tutorial!
=============================

.. sidebar:: Getting started with Zope Page Templates

  You can find introductions and more information about Zope Page
  Templates (ZPT, sometimes also called TAL) in various places:

    http://plone.org/documentation/tutorial/zpt

    http://wiki.zope.org/ZPT/FrontPage

  Note that some of the information in these introductions may refer
  to concepts not available in Grok or the Zope Toolkit, in particular
  variables like ``here`` or ``template``. The basic principles will
  work with Grok however; try reading ``context`` or ``view`` instead.

Grok is a powerful and flexible web application framework for Python
developers. In this tutorial we will show you the various things you
can do with Grok, and how Grok can help you build your web
application. We'll start out simple, and will slowly go to more
complex usage patterns.

All you're expected to know is the Python programming language and an
understanding of basic web programming (HTML, forms, URLs). It also
helps if you are familiar with Zope Page Templates, though most of the
examples should be fairly obvious if you are already familiar with
another templating language.

We recommend that beginners follow the tutorial from top to bottom. The
tutorial is designed to explain important concepts in order and slowly
builds up from there.

If you are more experienced, or just curious, you may want to skip
around instead and read the pieces which interest you most. If
something is unclear, you can always backtrack to previous sections.

Grok is based on the `Zope Toolkit`_. You do not need to know about
the Zope Toolkit at all to follow this tutorial.  Grok builds on
existing Zope Toolkit technology but exposes it in a special way to
the developer. We believe Grok makes developing with Zope Toolkit
technology easier and more fun for beginners and experienced
developers alike.

.. _`Zope Toolkit`: http://docs.zope.org/zopetoolkit/


Getting started with Grok
=========================

This chapter will help you get up and running with Grok, using the
``grokproject`` tool. We create a new project with ``grokproject`` and
tell you how to get that project running so you can access it with a
web browser.

Setting up grokproject
----------------------

.. sidebar:: Installing ``easy_install``

  If you don't already have ``easy_install`` available, you can find the
  script to set it up on the `PEAK EasyInstall page`_.

  .. _`PEAK EasyInstall page`: http://peak.telecommunity.com/DevCenter/EasyInstall#installing-easy-install

  You need to download `ez_setup.py`_. Then, you run it like this to
  install ``easy_install`` into your system Python::

    $ sudo python2.6 ez_setup.py

  .. _`ez_setup.py`: http://peak.telecommunity.com/dist/ez_setup.py

  This will make ``easy_install`` available to you.

  **Note**: Sometimes you have ``easy_install`` installed but you need
  a newer version of the underlying setuptools infrastructure to make
  Grok work. You can upgrade setuptools with::

    $ sudo easy_install -U setuptools

  **Note**: it is recommended you set up ``easy_install`` in a special
  virtualenv to isolate yourself from the system Python and whichever
  libraries it may have installed. This is especially relevant on the
  Mac OS X platform. See the next sidebar for more information.

.. sidebar:: Setting up a virtualenv

  Virtualenv is a tool that allows you to isolate your Python
  development environment entirely from the system and globally
  installed Python libraries. 

  On platforms like Mac OS X the use of virtualenv is especially
  recommended as some older versions of libraries (notably
  ``zope.interface``) are installed for the operation of Mac OS X
  services. Grok needs a newer version of ``zope.interface`` however,
  resulting in a conflict. Isolating yourself from the system Python
  is also recommended on Linux environments however, as Python is
  usually installed with the system package manager.

  If you do not want to use virtualenv it is always also possible to
  compile and install a different version of Python locally just for
  the use with Grok.

  During Grok installation on Linux and Mac OS X various libraries
  with C-level components are automatically compiled for you. On Linux
  you need to make sure you have the Python development headers
  installed; on Debian and Ubuntu they are in the ``python-dev``
  package. (use ``python2.6-dev`` to get the version for Python 2.6).

  These instructions are written for a Unix-style system and will be
  harder to follow on Windows. On Windows environments you can skip
  this step if you know you've installed a clean Python environment
  yourself. If however you intend to install a lot of software that
  uses this same version of Python, virtualenv is still recommended.

  You can install virtualenv with ``easy_install``::

    $ easy_install-2.6 virtualenv

  The ``virtualenv`` command-line tool should now be available to
  you. You can now create a sandbox environment for the use wih Grok::

    $ virtualenv --no-site-packages virtualgrok

  This will create a ``virtualgrok`` directory in your present
  location that contains the virtual environment.

  The ``--no-site-packages`` switch is important; it isolates your
  virtual Python environment from any packages installed in the system
  libraries.
 
  You should now activate the virtual environment::

    $ source virtualgrok/bin/activate
  
  Once you have activated the virtual environment, you can use it to
  ``easy_install`` grokproject in the regular way::

    $ 	
 
  Note that once you have created a Grok project from a virtual
  environment once you do not need to activate the virtual environment
  again afterwards -- the Grok project will know to use the special
  virtualgrok Python environment automatically. You only need to
  activate the ``grokproject`` virtualenv when you want to use the
  ``grokproject`` tool directly.

  For more information, see `Using Virtualenv for a clean Grok
  installation`_.

  .. _virtualenv: http://pypi.python.org/pypi/virtualenv

  .. _`Using Virtualenv for a clean Grok installation`: http://grok.zope.org/documentation/how-to/using-virtualenv-for-a-clean-grok-installation

Setting up grok on a Unix-like (Linux, Mac OS X) environment is
easy. Most of these instructions should also work in a Windows
environment as well.

Let's go through the prerequisites first. You need a computer
connected to the internet, as Grok installs itself over the
network. You also need Python 2.6 (or Python 2.5) installed.

Because Grok uses a source distribution of the Zope Toolkit libraries,
you may need to install your operating system's Python "dev" package
(``python-dev`` on Debian and Ubuntu, ``python2.6-dev`` for Python
2.6). You also need a working C compiler (typically ``gcc``)
installed, as we compile bits of the Zope Toolkit during setup
(``build-essential`` on Debian and Ubuntu). On Windows such a build
environment is not necessary, as grokproject will download and
automatically install precompiled libraries for Windows. Finally, you
also need ``easy_install`` installed so it becomes easy to install
Python packages.

Once you are done with the prerequisites, you can install
grokproject itself::

  $ easy_install grokproject

If you are on a Unixy environment and you are not working from the
recommended virtualenv, you will need to request admin rights using
``sudo``, as this will install new libraries into your system Python.

We're ready to create our first grok project now!

Creating a grok project
-----------------------

.. sidebar:: Using paster

  For those who know paster_: ``grokproject`` is just a wrapper around
  a paster template.  So instead of running the ``grokproject``
  command, you can also run::

  $ paster create -t grok Sample

  .. _paster: http://pythonpaste.org/script/

Let's create a first Grok project. A Grok project is a working
environment for a developer using Grok. In essence, a directory with a
lot of files and subdirectories in it. Let's create a Grok project
called Sample::

  $ grokproject Sample

.. sidebar:: Installing the previous 'zopectl' layout

  Grok used to have a different layout using ``zopectl``. To install
  this older layout, use ``grokproject`` with the ``--ctl`` flag::
  
  $ grokproject --zopectl Sample

This tells grokproject to create a new subdirectory called ``Sample``
and set up the project in there. grokproject will automatically
download and install the Zope Toolkit libraries as well as Grok into
the project area.

Grok asks you for an initial username and password for the
server. We'll use ``grok`` for both::

  Enter user (Name of an initial administrator user): grok
  Enter passwd (Password for the initial administrator user): grok

Now you have to wait while grokproject downloads and installs the
various tools and libraries that are needed in a Grok project. The
second time you create a Grok project it will be faster as it can
reuse the previously installed libraries. After all that your Grok
project is ready to go.

.. sidebar:: Common problems installing Grok

  *Library mixup*

  One common problem when installing Grok is library mixup. You may
  have some libraries installed in your Python interpreter that
  conflict with libraries that Grok wants to install. You tend to get
  an error when starting up the Grok web server when this is the
  case. If you already have installed Zope Toolkit libraries (or Zope
  3) previously for instance, you may first have to remove these
  libraries from your Python's ``site-packages`` directory. 

  Better yet, see the previous ``virtualenv`` sidebar for a way to
  isolate Grok from the system Python environment and its libraries,
  avoiding such problems.

  *No Python development environment*

  Grok includes dependencies that need to be compiled against Python.
  This happens automatically in the installation process, but a Python
  development environment does need to be installed, unless you are on
  Windows, where we supply binary versions of the required libraries.

  On Debian and Ubuntu this is the ``python-dev`` package
  (``python2.5-dev for Python 2.5). You also need ``build-essential``.

Starting up the web server
--------------------------

.. sidebar:: Run a Grok instance created with the older 'zopectl' layout

  To start up the Grok instance created with the older 'zopectl'
  layout::
  
  $ cd Sample
  $ bin/zopectl fg

  On Windows to work with ``zopectl`` you need to make sure you have
  win32all_ installed in your Python. It is not required to install
  win32all to work with the default ``paster`` setup.

  .. _win32all: http://sourceforge.net/projects/pywin32/

You can go into the ``Sample`` project directory now and start up the
web server for our project::

  $ cd Sample
  $ bin/paster serve parts/etc/deploy.ini

This will make Grok available on port 8080. You can log in with
username ``grok`` and password ``grok``. Assuming you've started up
the web server on your development computer, you can go to it here:

  	

This first pops up a login dialog (username: ``grok`` and password:
``grok``). It will then show a simple Grok admin interface. This user
interface allows you to install new Grok applications.

Our sample application (``sample.app.Sample``) will be available for
adding. Let's try this out.  Go to the Grok admin page:

  http://localhost:8080

and add a Sample application. Give it the name ``test``.

You can now go to the installed application if you click on its link. This
will bring you to the following URL:

  http://localhost:8080/test

You should see a simple web page with the following text on it::

  Congratulations!

  Your Grok application is up and running. Edit
  sample/app_templates/index.pt to change this page.

You can shut down the server at any time by hitting ``CTRL-c``. Shut
it down now. We will be shutting down and starting up the server often
in this tutorial.

Practice restarting the server now, as you'll end up doing it a lot
during this tutorial. It's just stopping and starting it again:
``CTRL-c`` and then ``bin/paster serve parts/etc/deploy.ini`` from
your Sample project directory. 

Alternatively, you can use the --reload flag to start up paster with a
monitor that scans your code base (python files only) for changes and
automatically restarts the server every time you make a change::

  $ bin/paster serve --reload parts/etc/deploy.ini

An empty Grok project
---------------------

.. sidebar:: What about the other directories and files in our project?

  What about the other files and subdirectories in our ``Sample``
  project directory? Grokproject sets up the project using a system
  called `zc.buildout`_. The ``eggs``, ``develop-eggs``, ``bin`` and
  ``parts`` directories are all set up and maintained by
  zc.buildout. See its documentation for more information about how to
  use it. The configuration of the project and its dependency is in
  ``buildout.cfg``. For now, you can avoid these details however.

  .. _`zc.buildout`: http://buildout.org

Let's take a closer look at what's been created in the Sample project
directory.

One of the things grokproject created was a ``setup.py`` file. This
file contains information about your project. This information is used
by buildout to download your project's dependencies and to install
it. You can also use the ``setup.py`` file to upload your project to
the Python Package Index (PyPI).

We have already seen the ``bin`` directory. It contains the startup
script for the web server (``bin/paster``) as well as the executable
for the buildout system (``bin/buildout``) which can be used to
re-build your project (to update it or to install a new dependency).

The ``parts` directory contain configuration and data created and
managed by ``buildout``and the ``.ini`` files to be used with 
``paster``. ``buildout`` also created the ``var`` directory, which
contains the Zope object database (ZODB) storage.

The actual code of the project will all be inside the ``src``
directory. In it is a Python package directory called ``sample`` with
the ``app.py`` file that grokproject said it would create. Let's look
at this file:

.. include:: groktut/an_empty_grok_project/src/sample/app.py
   :literal:

Not very much yet, but enough to make an installable Grok application
and display its welcome page. We'll go into the details of what this
means later.

Besides this, there is an empty ``__init__.py`` file to make this
directory a Python package. 

There is also a directory called ``app_templates``. It contains a single
template called ``index.pt``:

.. include:: groktut/an_empty_grok_project/src/sample/app_templates/index.pt
  :literal:

This is the template for your project's welcome page.

There is also a ``configure.zcml`` file. This file will normally only
contain a few lines that load dependencies and then register this
application with Grok. This means we can typically completely ignore
it, but we'll show it here once for good measure:

.. include:: groktut/an_empty_grok_project/src/sample/configure.zcml
   :literal:

.. sidebar:: ``configure.zcml`` in non-Grok applications

  In non-Grok applications that use the Zope Toolkit (such as
  something created with Zope 2 or Zope 3), the ZCML file usually
  plays a much bigger role. It contains directives which registers
  particular Python objects (typically classes, such as views) with
  the component architecture that is central to the Zope Toolkit.
  Grok however automates this registration by putting more information
  into the Python code directly, so that the ZCML file can remain small.

There is also a ``static`` directory. This contains static files that
can be used in the web application, such as images, css files and
javascript files.

Besides these files, there is an ``app.txt``, ``ftesting.zcml`` and
``tests.py``. These all have to do with the automatic testing
environment and can be ignored for now.

Showing pages
=============

Showing web pages is what puts the *web* in "web
applications". Typically HTML templates are used for this, but Grok
doesn't stop at templates. Most web pages in a real-world web
application will contain complex presentation logic that is better
handled by separate Python code in conjunction with templates. This
becomes especially important in more complex interactions with the
user, such as form handling. After reading this chapter, you should
already be able to write simple web applications with Grok.

Publishing a simple web page
----------------------------

Let's publish a simple static web page. Grok is geared towards web
applications and therefore is not really meant for publishing a large
number of static (pregenerated) web pages. For that you're better off
to use a specialized system such as Apache. Nonetheless, in order to
develop any web application we need to know how to put some simple
HTML on the web.

As you saw previously, our ``Sample`` application has a stock front
page, generated by grokproject. Let's change that.

To do this, go to the ``app_templates`` directory in ``src/sample/``.
This directory contains the templates used for anything defined in the
``app`` module. Grok knows to associate the directory to the module by
its name (``<module_name>_templates``).

In this directory we will edit the ``index`` template for our
``Sample`` application object. To do this, open the ``index.pt`` file
in a text editor. The ``.pt`` extension indicates that this file is a
Zope Page Template (ZPT). We're just going to put HTML in it now, but
this allows us to make page dynamic later on.

Change the ``index.pt`` file to contain the following (very
simplistic) HTML:

.. include:: groktut/publishing_a_simple_web_page/src/sample/app_templates/index.pt
  :literal:

Then reload the page:

  http://localhost:8080/test

You should now see the following text::

  Hello world!

Note that you can change templates and see the effects instantly:
there is no need to restart the web server to see the effect. This is
not true for changes on the Python level, for instance when you add a
template. We show an example of this next.

A second view
-------------

Our view is named ``index``. This in fact means something slightly
special: it's the default view of our application object. We can also
access it explicitly by naming the view:

  http://localhost:8080/test/index

If you view that URL in your browser, you should see the same result
as before. This is the way all other, non-index views are accessed.

Often, your application needs more than one view. A document for
instance may have an ``index`` view that displays it, but another
``edit`` view to change its contents.  To create a second view, create
another template called ``bye.pt`` in ``app_templates``. Make it have
the following content:

.. include:: groktut/a_second_view/src/sample/app_templates/bye.pt
  :literal:

Now we need to tell Grok to actually use this template. To do this,
modify ``src/sample/app.py`` so that it reads like this:

.. include:: groktut/a_second_view/src/sample/app.py
  :literal:

As you can see, all we did was add a class called ``Bye`` that
subclasses from ``grok.View``. This indicates to Grok that we want a
view named ``bye`` for the application, just like the ``Index`` class
that was already created for us indicates that we want a view named
``index``. A *view* is a way to view some model, in this case
installations of our ``Sample`` application. Note that the view name
in the URL is always going to be lowercase, while the class name
normally starts with an uppercase letter.

The empty class definition above is enough for Grok to go look in the
``app_templates`` directory for ``bye.pt``. The rule is that a the
template should have the same name as the class, but lowercased and
with the ``.pt`` postfix. 

.. sidebar:: Other templating languages

  You can also install extensions to allow the use of other templating
  languages in Grok, see for instance `megrok.genshi`_.

  .. _`megrok.genshi`: http://pypi.python.org/pypi/megrok.genshi/

Restart the web server (``CTRL-C``, then ``bin/paster serve
parts/etc/deploy.ini``). You can now go to a new web page called
``bye``:

  http://localhost:8080/test/bye

When you load this web page in a browser, you should see the following
text::

  Bye world!

Making our page dynamic
-----------------------

Static web pages are not very helpful if we want to make a dynamic web
application. Let's make a page that shows the result of a very simple
calculation: ``1 + 1``. 

We will use a Zope Page Templates (ZPT) directive to do this
calculation inside ``index.pt`` template. Change the ``index.pt`` to
read like this:

.. include:: groktut/making_our_page_dynamic/src/sample/app_templates/index.pt
  :literal:

We've used the ``tal:content`` page template directive to replace the
content between the ``<p>`` and ``</p>`` tags with something else, in
this case the result of the Python expression ``1 + 1``.

Since restarting the server is not necessary for changes that are
limited to the page templates, you can just reload the web page:

  http://localhost:8080/test

You should see the following result::

  2

Looking at the source of the web page shows us this::

  <html>
  <body>
  <p>2</p>
  </body>
  </html>

As you can see, the content of the ``<p>`` tag was indeed replaced
with the result of the expression ``1 + 1``.

Static resources for our web page
---------------------------------

In real-world web pages, we almost never publish a web page that just
contains bare-bones HTML. We also want to refer to other resources,
such as images, CSS files or Javascript files. As an example, let's
add some style to our web page.

To do this, create a new directory called ``static`` in the ``sample``
package (so, ``src/sample/static``). In it, place a file called
``style.css`` and put in the following content:

.. include:: groktut/static_resources_for_our_web_page/src/sample/static/style.css
  :literal:

In order to use it, we also need to refer to it from our
``index.pt``. Change the content of ``index.pt`` to read like this:

.. include:: groktut/static_resources_for_our_web_page/src/sample/app_templates/index.pt
  :literal:

Now restart the server and reload the page:

  http://localhost:8080/test

The web page should now show up with a red background.

You will have noticed we used the ``tal:attributes`` directive in our
``index.pt`` now. This uses Zope Page Templates to dynamically
generate the link to our file ``style.css``.

Let's take a look at the source code of the generated web page::

  <html>
  <link rel="stylesheet" type="text/css"
        href="http://localhost:8080/test/@@/sample/style.css" />
  <body>
  <p>Hello world!</p>
  </body>
  </html>

As you can see, the ``tal:attributes`` directive is gone and has been
replaced with the following URL to the actual stylesheet:

  http://localhost:8080/test/@@/sample/style.css

We will not go into the details of the structure of the URL here, but
we will note that because it's generated this way, the link to
``style.css`` will continue to work no matter where you install your
application (i.e. in a virtual hosting setup).

Pulling in images or javascript is very similar. Just place your image
files and `.js` files in the ``static`` directory, and create the URL
to them using ``static/<filename>`` in your page template.

Using view methods
------------------

.. sidebar:: Unassociated templates

  If you have followed the tutorial so far, you will have an extra
  template called ``bye.pt`` in your ``app_templates`` directory.
  Since in the given ``app.py`` we we have no more class using it, the
  ``bye.pt`` template will have become *unassociated*. When you try to
  restart the server, Grok will give you a warning like this::

    UserWarning: Found the following unassociated template(s) when
    grokking 'sample.app': bye.  Define view classes inheriting from
    grok.View to enable the template(s).

  To get rid of this warning, simply remove ``bye.pt`` from your
  ``app_templates`` directory.

ZPT is deliberately limited in what it allows you to do with Python.
It is good application design practice to use ZPT for fairly simple
templating purposes only, and to do anything a bit more complicated in
Python code. Using ZPT with arbitrary Python code is easy: you just
add methods to your view class and use them from your template.

Let's see how this is done by making a web page that displays the
current date and time. We will use our Python interpreter to find out
what works::

  $ python
  Python 2.6.4
  Type "help", "copyright", "credits" or "license" for more information.
  >>> 

We will need Python's ``datetime`` class, so let's import it::

  >>> from datetime import datetime

Note that this statement brings us beyond the capabilities of simple
ZPT use; it is not allowed to import arbitrary Python modules from
within a ZPT template; only Python *expressions* (with a result) are
allowed, not *statements* such as ``from .. import ..``.

Let's get the current date and time::

  >>> now = datetime.now()

This gives us a date time object; something like this::

  >>> now
  datetime.datetime(2010, 2, 26, 15, 38, 41, 259245)

Not very nice to display on a web page, so let's turn it into a
prettier string using the formatting capabilities of the ``datetime``
object::

  >>> now.strftime('%Y-%m-%d %H:%M')
  '2010-02-26 15:38'

That looks better.

So far nothing new; just Python. We will integrate this code into our
Grok project now. Go to ``app.py`` and change it to read like this:

.. include:: groktut/using_view_methods/src/sample/app.py
  :literal:

We've simply added a method to our view that returns a string
representing the current date and time. Now to get this string in our
page template. Change ``index.pt`` to read like this:

.. include:: groktut/using_view_methods/src/sample/app_templates/index.pt
  :literal:

Restart the server. This is needed as we changed the content of a Python
file (``app.py``). Now reload our index page to see whether it worked:

  http://localhost:8080/test

You should see a web page with a date and time like this on your
screen now::

  2010-02-26 15:39

What happened here? When viewing a page, the view class (in this case
``Index`` is instantiated by the framework. The name ``view`` in the
template is always made available and is associated with this
instance. We then simply call the method on it in our template.

There is another way to write the template that is slightly shorter
and may be easier to read in some cases, using a ZPT path expression::

  <html>
  <body>
  <p tal:content="view/current_datetime"></p>
  </body>
  </html>

Running this has the same result as before.

Generating HTML from Python
---------------------------

While usually you will be using templates to generate HTML, sometimes
you want to generate complicated HTML in Python and then include it in
an existing web page. For reasons of security against cross-site
scripting attacks, TAL will automatically escape any HTML into `&gt;`
and `&lt;`. With the ``structure`` directive, you can tell TAL
explicitly to not escape HTML this way, so it is passed literally into
the template. Let's see how this is done. Change ``app.pt`` to read like
this:

.. include:: groktut/generating_html_from_python/src/sample/app.py
  :literal:

and then change ``index.pt`` to read like the following:

.. include:: groktut/generating_html_from_python/src/sample/app_templates/index.pt
  :literal:

Let's take another look at our web page:

  http://localhost:8080/test

You should see the following text (in bold):

  **ME GROK BOLD**

This means the HTML we generated from the ``some_html`` method was
indeed successfully integrated in our web page.  Without the the
``structure`` directive, you would've seen the following instead::
 
  <b>ME GROK BOLD</b>

Completely Python-driven views
------------------------------

.. sidebar:: Setting the content-type

  When generating the complete content of a page yourself, it's often
  useful to change the content-type of the page to something else than
  ``text/plain``. Let's change our code to return simple XML and set
  the content type to ``text/xml``:

  .. include:: groktut/setting_the_content_type/src/sample/app.py
    :literal:

  All views in Grok have a ``response`` property that you can use to
  manipulate response headers.

Sometimes it is inconvenient to have to use a template at all. Perhaps
we are not returning a HTML page at all, for instance. In this case, we
can use the special ``render`` method on a view.

Modify ``app.py`` so it reads like this:

.. include:: groktut/completely_python_driven_views/src/sample/app.py
  :literal:

If you were to start up the server with an ``index.pt`` template still
inside ``app_templates`` you would get a an error::

    GrokError: Multiple possible ways to render view <class
    'sample.app.Index'>. It has both a 'render' method as well as an
    associated template.

In the face of ambiguity Grok, like Python, refuses to guess. To
resolve this error, remove ``index.pt`` from the ``app_templates``
directory.

Now take another look at our test application:

  http://localhost:8080/test

You should see the following::

  ME GROK NO TEMPLATE

You should see this even when you view the source of the page. When
looking at the content type of this page, you will see that it is
``text/plain``.

Doing some calculation before viewing a page
--------------------------------------------

Instead of calculating some values in a method call from the template,
it is often more useful to calculate just before the web page's
template is calculated. This way you are sure that a value is only
calculated once per view, even if you use it multiple times.

You can do this by defining an ``update`` method on the view class. Modify
``app.py`` to read like this:

.. include:: groktut/doing_some_calculation_before_viewing_a_page/src/sample/app.py
  :literal:

This sets a name ``alpha`` on the view just before the template is
being displayed, so we can use it from the template. You can set as
many names on ``self`` as you like.

Now we need a template ``index.pt`` that uses ``alpha``:

.. include:: groktut/doing_some_calculation_before_viewing_a_page/src/sample/app_templates/index.pt
  :literal:

Restart the server and then let's take another look at our application:

  http://localhost:8080/test

You should see 256, which is indeed 2 raised to the power 8.

Reading URL parameters
----------------------

When developing a web application, you don't just want to output data,
but also want to use input. One of the simplest ways for a web
application to receive input is by retrieving information as a URL
parameter. Let's devise a web application that can do sums for us. In
this application, if you enter the following URL into that
application:

  http://localhost:8080/test?value1=3&value2=5

you should see the sum (8) as the result on the page. 

Modify ``app.py`` to read like this:

.. include:: groktut/reading_url_parameters/src/sample/app.py
  :literal:

We need an ``index.pt`` that uses ``sum``:

.. include:: groktut/reading_url_parameters/src/sample/app_templates/index.pt
  :literal:

Restart the server. Now going to the following URL should display 8:

  http://localhost:8080/test?value1=3&value2=5

Other sums work too, of course:

  http://localhost:8080/test?value1=50&value2=50

What if we don't supply the needed parameters (``value1`` and
``value2``) to the request? We get an error:

  http://localhost:8080/test

You can look at the window where you started up the server to see the error
traceback. This is the relevant complaint::

  TypeError: Missing argument to update(): value1

We can modify our code so it works even without input for either parameter:

.. include:: groktut/reading_url_parameters2/src/sample/app.py
  :literal:

Restart the server, and see it can now deal with missing parameters (they
default to ``0``).

Simple forms
------------

.. sidebar:: Automatic forms

  Creating forms and converting and validating user input by hand, as
  shown in this section, can be rather cumbersome. With Grok, you can
  use the Zope Toolkit's *schema* and *formlib* systems to automate
  this and more. This will be discussed in a later section. TDB

Entering the parameters through URLs is not very pretty. Let's use a
form for this instead. Change ``index.pt`` to contain a form, like
this:

.. include:: groktut/simple_forms/src/sample/app_templates/index.pt
  :literal:

One thing to note here is that we dynamically generate the form's
``action``. We make the form submit to itself, basically. Grok views
have a special method called ``url`` that you can use to retrieve the
URL of the view itself (and other URLs which we'll go into later).

Leave the ``app.py`` as in the previous section, for now. You can now
go to the web page::

  http://localhost:8080/test

You can submit the form with some values, and see the result displayed
below.

We still have a few bugs to deal with however. For one, if we don't fill
in any parameters and submit the form, we get an error like this::

  File "../app.py", line 8, in update
    self.sum = int(value1) + int(value2)
  ValueError: invalid literal for int(): 

This is because the parameters were empty strings, which cannot be
converted to integers. Another thing that is not really pretty is that
it displays a sum (0) even if we did not enter any data. Let's change
``app.py`` to take both cases into account:

.. include:: groktut/simple_forms2/src/sample/app.py
  :literal:

We catch any TypeError and ValueError here so that wrong or missing
data does not result in a failure. Instead we display the text "No
sum". If we don't get any error, the conversion to integer was fine,
and we can display the sum.

Restart the server and go to the form again to try it out:

  http://localhost:8080/test

Models
======

Now we know how to show web pages, we need to go into what we are
actually showing: the models. The models contain the
display-independent logic of your application. In this chapter we will
discuss a number of issues surrounding models: how your views connect
to models, and how you can make sure the data in your models is stored
safely. As the complexity of our sample applications grows, we will
also go into a few more issues surrounding form handling.

A view for a model
------------------

So far, we have only seen views that do the work all by themselves.
In typical applications this is not the case however - views display
information that is stored elsewhere. In Grok applications, views work
for models: subclasses of ``grok.Model`` or ``grok.Container``. For
the purposes of this discussion, we can treat a ``grok.Container`` as
another kind of ``grok.Model`` (more about what makes
``grok.Container`` special later XXX).

Our ``Sample`` class is a ``grok.Container``, so let's use ``Sample``
to demonstrate the basic principle. Let's modify ``app.py`` so that
``Sample`` actually makes some data available:

.. include:: groktut/a_view_for_a_model/src/sample/app.py
  :literal:

In this case, the information (``"This is important information!"``)
is just hardcoded, but you can imagine information is retrieved from
somewhere else, such as a relational database or the filesystem.

We now want to display this information in our template ``index.pt``:

.. include:: groktut/a_view_for_a_model/src/sample/app_templates/index.pt
  :literal:

Restart the server. When you view the page:

  http://localhost:8080/test

You should now see the following::

  This is important information!

Previously we have seen that you can access methods and attributes on
the view using the special ``view`` name in a template. Similarly, the
name ``context`` is also available in each template. ``context``
allows us to access information on the context object the view is
displaying. In this case this is an instance of ``Sample``, our
application object.

Separating the model from the view that displays it is an important
concept in structuring applications. The view, along with the
template, is responsible for displaying the information and its user
interface. The model represents the actual information (or content)
the application is about, such as documents, blog entries or wiki
pages. The model should not know anything about the way it is
displayed.

This way of structuring your applications allows you to change the way
your model is displayed without modifying the model itself, just
the way it is viewed.

Let's do that by making the view do something to the information. Change 
``app.py`` again:

.. include:: groktut/a_view_for_a_model2/src/sample/app.py
  :literal:

You can see that it is possible to access the context object (an
instance of ``Sample``) from within the view class, by accessing the
``context`` attribute. This gets the same object as when we used the
``context`` name in our template before.

What we do here is reverse the string returned from the
``information()`` method. You can try it on the Python prompt::

  >>> ''.join(reversed('foo'))
  'oof'

Now let's modify the ``index.pt`` template so that it uses the
``reversed_information`` method:

.. include:: groktut/a_view_for_a_model2/src/sample/app_templates/index.pt
  :literal:

Restart the server. When you view the page:

  http://localhost:8080/test

You should now see the following:

  The information: This is important information!

  The information, reversed: !noitamrofni tnatropmi si sihT 

Storing data
------------

So far we have only displayed either hardcoded data, or calculations
based on end-user input. What if we actually want to *store* some
information, such as something the user entered? The easiest way to do
this with Grok is to use the Zope Object Database (ZODB).

The ZODB is a database of Python objects. You can store any Python
object in it, though you do need to follow a few simple rules (the
"rules of persistence", which we will go into later). Our ``Sample``
application object is stored in the object database, so we can store
some information on it.

Let's create an application that stores a bit of text for us. We will
use one view to view the text (``index``) and another to edit it
(``edit``).

Modify ``app.py`` to read like this:

.. include:: groktut/storing_data/src/sample/app.py
  :literal:

The ``Sample`` class gained a class attribute with some default text.
In the ``update`` method of the ``Edit`` view you can see we actually
set the ``text`` attribute on the context, if at least a ``text``
value was supplied by a form. This will set the ``text`` attribute on
the instance of the ``Sample`` object in the object database, and thus
will override the default ``text`` class attribute.

Change the ``index.pt`` template to read like this:

.. include:: groktut/storing_data/src/sample/app_templates/index.pt
  :literal:

This is a very simple template that just displays the ``text``
attribute of the ``context`` object (our ``Sample`` instance).

Create an ``edit.pt`` template with the following content:

.. include:: groktut/storing_data/src/sample/app_templates/edit.pt
  :literal:

This template display a form asking for a bit of text. It submits to
itself.

Restart the server. Let's first view the index page:

  http://localhost:8080/test

You should see ``default text``.

Now let's modify the text by doing to the edit page of the application:

  http://localhost:8080/test/edit

Type in some text and press the "Store" button. Since it submits to
itself, we will see the form again, so go to the index page manually:

  http://localhost:8080/test
 
You should now see the text you just entered on the page. This means
that your text was successfully stored in the object database!

You can even restart the server and go back to the index page, and your text
should still be there.

Redirection
-----------

Let's make our application a bit easier to use. First, let's change
``index.pt`` so it includes a link to the edit page. To do this, we
will use the ``url`` method on the view:

.. include:: groktut/redirection/src/sample/app_templates/index.pt
  :literal:

Giving ``url`` a single string argument will generate a URL to the
view named that way on the same object (``test``), so in this case
``test/edit``.

Now let's change the edit form so that it redirects back to the
``index`` page after you press the submit button:

.. include:: groktut/redirection/src/sample/app.py
  :literal:

The last line is the new one. We use the ``url`` method on the view to
construct a URL to the ``index`` page. Since we're in the template, we
can simply call ``url`` on ``self``. Then, we pass this to another
special method available on all ``grok.View`` subclasses,
``redirect``. We tell the system to redirect to the ``index`` page.

Showing the value in the form
-----------------------------

Let's change our application so it displays what we stored the edit
form as well, not just on the index page.

To make this work, change edit.pt so it reads like this:

.. include:: groktut/showing_the_value_in_the_form/src/sample/app_templates/edit.pt
  :literal:

The only change is that we have used ``tal:attributes`` to include the
value of the ``text`` attribute of the context object in the form.

The rules of persistence
------------------------

These are the "rules of persistence":

* You should subclass classes that want to store data from
  ``persistent.Persistent`` so that it's easy to store them in the
  ZODB. The simplest way to do this with Grok is to subclass from
  ``grok.Model`` or ``grok.Container``.

* Instances that you want to store should be connected to other
  persistent classes that are already stored. The simplest way to do
  this with Grok is to attach them somehow to the ``grok.Application``
  object, directly or indirectly. This can be done by setting them as
  an attribute, or by putting them in a container (if you made your
  application subclass ``grok.Container``).

* To make sure that the ZODB knows you changed a mutable attribute
  (such as a simple Python list or dictionary) in your instance, set
  the special ``_p_changed`` attribute on that instance to
  ``True``. This is only necessary if that attribute is not
  ``Persistent`` itself. It is also not necessary when you create or
  overwrite an attribute directly using ``=``.

If you construct your application's content out of ``grok.Model`` and
``grok.Container`` subclasses you mostly follow the rules
already. Just remember to set ``_p_changed`` in your methods if you
find yourself modifying a Python list (with ``append``, for instance)
or dictionary (by storing a value in it).

The code in the section `Storing data`_ is a simple example. We in
fact have to do nothing special at all to obey the rules of
persistence in that case.

If we use a mutable object such as a list or dictionary to store data
instead, we do need to take special action. Let's change our example
code (based on the last section) to use a mutable object (a list):

.. include:: groktut/the_rules_of_persistence/src/sample/app.py
  :literal:

We have now changed the ``Sample`` class to do something new: it has
an ``__init__`` method. Whenever you create the ``Sample`` application
object now, it will be created with an attribute called ``list``,
which will contain an empty Python list. 

We also make sure that the ``__init__`` method of the superclass still
gets executed, by using the regular Python ``super`` idiom. If we
didn't do that, our container would not be fully initialized.

You will also notice a small change to the ``update`` method of the
``Edit`` class. Instead of just storing the text as an attribute of
our ``Sample`` model, we add each text we enter to the new
``list`` attribute on. 

Note that this code has a subtle bug in it, which is why we've added
the comment. We will see what bug this is in a little bit. First,
though, let's change our templates.

We change ``index.pt`` so that it displays the list:

.. include:: groktut/the_rules_of_persistence/src/sample/app_templates/index.pt
  :literal:

We've also changed the text of the link to the ``edit`` page to reflect
the new adding behavior of our application.

We need to undo the change to the ``edit.pt`` template that we
made in the last section, as each time we edit a text we now *add* a
new text, instead of changing the original. There is therefore no text
to show in as the input value anymore:

.. include:: groktut/the_rules_of_persistence/src/sample/app_templates/edit.pt
  :literal:

.. sidebar:: evolution

  What to do when you change an object's storage structure while your
  application is already in production? In a later section, we will
  introduce Zope Toolkit's object evolution mechanism that allows you to
  update objects in an existing object database. TDB

Let's restart the server. If you have followed the tutorial from the
last section, you will now see an error when you look at the front
page of the application::

  A system error occurred. 

Look at the output we got when we tried to load our page::

  AttributeError: 'Sample' object has no attribute 'list'

But we just changed our object to have an attribute ``list``, right?
Yes we did, but only for *new* instances of the Sample object. What we
are looking at is the sample object from before, still stored in the
object database. It has no such attribute. This isn't a bug by the way
(for our actual bug, see later in this section): it is just a database
problem.

What to do now? The simplest action to take during development is to
simply remove our previously installed application, and create a new
one that *does* have this attribute. Go to the Grok admin screen:

  http://localhost:8080

Select the application object (``test``) and delete it. Now install it
again, as ``test``. Now go to its edit screen and add a text:

  http://localhost:8080/test/edit

Click on ``add a text`` and add another text. You will see the new
texts appear on the ``index`` page.

Everything is just fine now, right? In fact, not so! Now we will get
to our bug. Restart the server and look at the index page again:

  http://localhost:8080/test

None of the texts we added were saved! What happened? We broke the
third rule of persistence as described above: we modified a mutable
attribute and did not notify the database that we made this
change. This means that the object database was not aware of our
change to the object in memory, and thus never saved it to disk.

.. sidebar: The ZODB only stores instance data

  Note that the ZODB only stores ("persists") instance data. This
  means that any data you have directly associated with a class, as
  opposed to the instance, won't be persisted. Normally you only
  associate immutable data with the class, so this is not a problem::
 
    class Foo(object):
        mydata = 'some text'

  That data will be there when the module is imported, and since it 
  will never be changed, there isn't a problem. Now let's check what
  happens with mutable data::

    class Foo(object):
        mydata = []

  Appending an item to mydata (through ``self.mydata.append('bar')``,
  for instance) have an effect, but only until you restart the
  server. Then your changes will be lost.

  It is good Python design practice not to use mutable class-data, so
  this property of the ZODB shouldn't cramp your style.
 
We can easily amend this by adding one line to the code:

.. include:: groktut/the_rules_of_persistence2/src/sample/app.py
  :literal:

We've now told the server that the context object has changed (because
we modified a mutable sub-object), by adding the line::

  self.context._p_changed = True

If you now add some texts and then restart the server, you will notice the
data is still there: it has successfully been stored in the object
database.

The code shown so far is a bit ugly in the sense that typically we
would want to manage our state in the model code (the ``Sample``
object in this case), and not in the view. Let's make one final
change to show what that would look like:

.. include:: groktut/the_rules_of_persistence3/src/sample/app.py
  :literal:

As you can see, we have created a method ``addText`` to the model that
takes care of amending the list and informing the ZODB about it. This
way, any view code can safely use the API of ``Sample`` without having
to worry about the rules of persistence itself, as that is the model's
responsibility.

Explicitly associating a view with a model
------------------------------------------

How does Grok know that a view belongs to a model? In the previous
examples, Grok has made this association automatically. Grok could do
this because there was only a single model defined in the module
(``Sample``). In this case, Grok is clever enough to automatically
associate all views defined elsewhere in the same module to the only
model. Behind the scenes Grok made the model the *context* of the
views.

Everything that Grok does implicitly you can also tell Grok to do
explicitly. This will come in handy later, as you may sometimes need
(or want) to tell Grok what to do, overriding its default behavior. To
associate a view with a model automatically, you use the
``grok.context`` class annotation.

What is a class annotation? A class annotation is a declarative way
to tell grok something about a Python class. Let's look at an example.
We will change ``app.py`` in the example from `A second view` to demonstrate
the use of ``grok.context``:

.. include:: groktut/explicitly_associating_a_view_with_a_model/src/sample/app.py
  :literal:

This code behaves in exactly the same way as the previous example in
`A second view`, but has the relationship between the model and the
view made explicit, using the ``grok.context`` class annotation.

``grok.context`` is just one class annotation out of many. We will see
another one (``grok.name``) in the next section.

A second model
--------------

.. sidebar:: How to combine models into a single application?

  Curious now about how to combine models into a single application?
  Can't wait? Look at the section `Containers` coming up next, or
  `Traversal` later on. TDB

We will now extend our application with a second model. Since we
haven't explained yet how to combine models together into a single
application, we will just create a second application next to our
first one. Normally we probably wouldn't want to define two
applications in the same module, but we are trying to illustrate a few
points, so please bear with us. Change ``app.py`` so it looks like
this:

.. include:: groktut/a_second_model/src/sample/app.py
  :literal:

You can see we now defined a second application class, ``Another``.
It subclasses from ``grok.Application`` to make it an installable
application.  

It also subclasses from ``grok.Model``. There is a difference between
``grok.Model`` and ``grok.Container``, but for the purpose of the
discussion we can ignore it for now. We just figured we should use
``grok.Model`` for some variety, though we could have indeed
subclassed from ``grok.Container`` instead.

We also define two templates, one called ``sampleindex.pt``:

.. include:: groktut/a_second_model/src/sample/app_templates/sampleindex.pt
  :literal:

And one called ``anotherindex.pt``:

.. include:: groktut/a_second_model/src/sample/app_templates/anotherindex.pt
  :literal:

We have named the templates the name as the lowercased class names as
the views, so that they get associated with them.

You will have noticed we have used ``grok.context`` to associate the
views with models. We actually *have* to do this here, as Grok refuses
to guess in the face of ambiguity. Without the use of
``grok.context``, we would have seen an error like this when we start
up::

  GrokError: Multiple possible contexts for <class
  'sample.app.AnotherIndex'>, please use grok.context.

So, we use ``grok.context`` to explicitly associate ``SampleIndex``
with the ``Sample`` application, and again to associate
``AnotherIndex`` with the ``Another`` application.

We have another problem: the intent is for these views to be ``index``
views. This cannot be deduced automatically from the name of the view
classes however, and left to its own devices, Grok would have called
the views ``sampleindex`` and ``anotherindex``. 

Luckily we have another class annotation that can help us here:
``grok.name``. We can use it on both view classes
(``grok.name('index')``) to explicitly explain to Grok what we want.

You can now try to restart the server and create both applications in
the Grok Admin interface. They should display the correct index pages
when you look at them.

We can see that the introduction of a second model has complicated our
code a bit, though you will hopefully agree with us that it is still
quite readable. We could have avoided the whole problem by simply
placing ``Another`` and its views in another module such as
``another.py``.  Its associated templates would then need to be placed
in a directory ``another_templates``. Often you will find it possible
to structure your application so you can use Grok's default
conventions.

Containers 
----------

A container is a special kind of model object that can contain other
objects. Our ``Sample`` application is already a container, as it
subclasses ``grok.Container``. What we will do in this section is
build an application that actually puts something into that container.

Grok applications are typically composed of containers and
models. Containers are objects that can contain models. This includes
other containers, as a container is just a special kind of model.

>From the perspective of Python, you can think of containers as
dictionaries.  They allow item access (``container['key']``) to get at
its contents. They also define methods such as ``keys()`` and
``values()``. Containers do a lot more than Python dictionaries
though: they are persistent, and when you modify them, you don't have
to use `_p_changed` anywhere to notice you changed them. They also
send out special events that you can listen to when items are placed
in them or removed from them. For more on that, see the section on
events (TDB).

Our application object will have a single index page that displays the
list of items in the container. You can click an item in the list to
view that item. Below the list, it will display a form that allows you
to create new items.

Here is the ``app.py`` of our new application:

.. include:: groktut/containers/src/sample/app.py
  :literal:

As you can see, ``Sample`` is unchanged. We have also created our
first non-application object, ``Entry``. It is just a
``grok.Model``. It needs to be created with an argument ``text`` and
this text is stored in it. We intend to place instances of ``Entry``
in our ``Sample`` container.

Next are the views. We have an ``index`` page for the ``Sample``
container. When its ``update()`` is triggered with two values,
``name`` and ``text``, it will create a new ``Entry`` instance with
the given text, and place it under the container under the name
``name``. We use the dictionary-like interface of our ``Sample``
container to put our new ``Entry`` in the container.

Here is the associated template for ``SampleIndex``, ``sampleindex.pt``:

.. include:: groktut/containers/src/sample/app_templates/sampleindex.pt
  :literal:

The first section in the template (``<h2>Existing entries</h2>``)
displays a list of the items in the container. We again use
dictionary-like access using ``keys()`` to get a list of all the names
of the items in the container. We create a link to these items using
``view.url()``.

The next section (``<h2>Add a new entry</h2>``) displays a simple form
that submits to the index page itself. It has two fields, ``name`` and
``text``, which we already have seen handled by ``update()``.

Finally, we have an ``index`` page for ``Entry``. It just has a template
to display the ``text`` attribute:

.. include:: groktut/containers/src/sample/app_templates/entryindex.pt
  :literal:

Restart the server and try this application.  Call your application
``test``. Pay special attention to the URLs.

First, we have the index page of our application:

  http://localhost:8080/test

When we create an entry called ``hello`` in the form, and then click on it
in the list, you see an URL that looks like this:

  http://localhost:8080/test/hello

We are now looking at the index page of the instance of ``Entry``
called ``hello``.

What kind of extensions to this application can we think of? We could
create an ``edit`` form that allows you to edit the text of
entries. We could modify our application so that you can not just add
instances of ``Entry``, but also other containers. If you made those
modifications, you would be on your way to building your own content
management system with Grok.
