Hey! What's time now?

Oh! it's only 10 minutes since we started reading this article. We got 20 more minutes to explain what's happened here. So let's look back again.

Installation and Configuration

You can download Zope 3.1.0 or later for your major works. Install Zope3 as root, though it is not necessary. When you experiment with Zope3, make instance as a normal user. For production systems, you get the additional advantage of OS level security. Since we manually installed Zope3 (as root), the default installation path will be /usr/local/Zope-3.1.x . When making zope instance, you have to specify zope manager's user name and password. You can make more instances, if required.

In your Zope 3 instance directory, there are few directories with specific purposes. Here ownwards I will be using $ZOPE3INSTANCE to refer your instance directory

$ZOPE3INSTANCE/etc

In 'etc' you can see some configuration files. In $ZOPE3INSTANCE/etc/zope.conf you can edit port numbers of http and ftp servers.

$ZOPE3INSTANCE/var

Zope saving all data here. Backup this directory regulary. If 'Data.fs' is missing, it will be automatically created when you run zope again.

$ZOPE3INSTANCE/bin

Few utility scripts. 'runzope' and 'test' are already familiar to you. 'pyskel' script will be very useful for creating template codes from interfaces, Example:

$ $ZOPE3INSTANCE/bin/pyskel  boom.interfaces.IMark > file.py

$ZOPE3INSTANCE/lib/python

As I said earlier you can place your Zope3 packages here. By default this won't be in your Python path. You can also place your package in any standard Python path.

Why should I code like this?

When we started discussing BookMarker, the first word I coined is Extreme Programming (XP). I strongly recommend you to read about Extreme Programming. It will really influence you to write code in Zope3 way (directly and indirectly). Ok, one causion: many XPers believe that frameworks are bad. Here, I can not give an answer in one sentence, so you find out yourself. "Extreme Programming Explained" by Kent Beck will be a good starting point for your study. By the way, another classic book which you can read is "Design Patterns" by Gang of Four. In this book they say : 'Program to an interface, not to an implementation'. To simulate a formal definition of interfaces in C++, they used classes with virtual functions. Just like this, in Python we will use zope.interface.Interfce inherited metaclass for defining an interface. There is every chance for an 'interface' concept in Python language by version 3.0 . According to Extreme Programming: 'The four basic activities of development are coding, testing, listening, and designing.' Zope3 makes your software testing, a breeze. You can write unit and functional tests very easily.

Let's look into interfaces

I hope you are familiar with the concept of interfaces. Anyway, when we speak about an interface, we meant it to use with reference to an object (i.e, an instance of a class). For example if 'A' is a class which implements 'IA', and 'a' is an instance of 'A', then 'IA' is the interface of 'a', got it? But normally we only deals with the interface and its implementing class.

In our interfaces.py we defined three interfaces. The first interface 'IMark' defines a book mark object. A book mark has two attributes, one is the url and another one is the description. We used schemas available at zope.schema package to specify these attributes.

As we mentioned earlier IBookMarker is a container interface. We extended IContainer interface with one name attribute. Also we put one item type pre-condition. So IBookMarker object can only contain an IMark object. Now before moving to the next section, please note from which packages and modules we imported different classes/interfaces. Just open the sources of those packages, and see how well documented they are!. Some documentation are written in seperate ReStructuredText files with unit testing, this is the Zope3 way of unit testing.

Unit testing re-visited

We wrote our test in a file named tests.py. You can also write tests in a package named tests. All test modules under this package should have test_ prefix. We also automated our doctests written along with implementation. To test our BookMarker container, we inherited TestSampleContainer. For testing containers, you can stick with unittest module. Also use it where you want more reusablity. Anyway, we will integrate all doctests with unittest module. This will help us to run tests automatically, from a single point.

Let's talk about implementations

In the first line of bookmarker.py we set a special variable attribute __docformat__ = 'restructuredtext'. Our documentation strings are written in ReStructuredText format. I strongly reccomend you to use ReST for all kinds of Python documentations. (This article is written in ReST: svn co svn://svn.berlios.de/zissue/trunk/z3in30m).

First we imported implements function from zope.interface package. Using this function we can say a class implements one or more interfaces. BTreeContainer is full implementation of IContainer interface. We inherit it to implement our Container interfaces. Similarly Contained is an implementation of IContained. Mark class implements both IMark and IMarkContained. IMarkContained is an extended interface of IContained. So by inheriting Contained class we get a partial implementation of our interfaces. Now the remaining things to implement is two attributes, url and description. Similarly majority of IBookMarker is implemented in BTreeContainer. I hope documentation strings are self explanatory.

ZCML Explained

Let me ask one question, which you are going to face many times later. Do you think a Programming language should be used to write configuration files? If yes, why?. Why we can not use a text files with some conventions or markups.

Anyway, ZCML is the XML based configuration system for Zope3. The base tag 'configure' specifies namespaces to use. In our configuration we used two namespaces, 'zope' and 'browser'. 'zope' namespace contains the basic elements required to register our content objects. 'browser' is for view related configuration.

The first tag we used is 'interface'. It is called a directive. In our configuration 'iterface' is a simple directive and 'content' is a complex directive.

Views and ZPT

Your objects are finally saved in ZODB, you may choose other storage mechanisms also. In zope you can present your objects in different ways. I mean, through different protocols like http, ftp, xmlrpc etc. In our case, we created a view for browser (yes, through http). We put all logics for presentation in browser.py. And created a Zope Page Template (ZPT) for real presentation. Normally you should avoid any information retrieval logic in ZPT.

The things we didn't mentioned

Please go to the last section, otherwise you can not finish this article in 30 minutes :)

Ofcourse, in an introductory document we can only cover a fraction of the whole technology. Here I have omitted many things for various reasons.

In the beginning, I said just one assumptiion about you. I really meant many other things. Surely you should be a Python programmer. You should know basic administration. Basic understanding of web technologies. And ofcourse, willing to spend time :)

While installation you might get error due to Python development libraries not installed. If you know Python and system administration you will just run: apt-get install python-dev Similary you have created __init__.py file to make our package working.

If you are interested in Zope3 by now, surely you will explore more in future. While you exploring, you will see that doctests can be written in stand alone text files. Similarly you will understand lots of other things.

One thing I can assure you is that zope is really a matured framework. It has almost 9 year history of successfull running. Many technologies evolved through zope. Infact zope and zope developers has influenced Python language itself. Yes! it is one of the gratest Python application ever written.

The End

Oh! it is just the end of this article. And the great beginning of your journey through Zope3 world. So, good luck.

About the author

I am Python programmer from Kerala. I love Python. Previously I have worked for Malayalam i18n & l10n in free softwares. I have worked for Free Software Foundation of India (as a job). I was a Koha consultant for some time. Currently I am doing lots of Python, PyGTK and PostgreSQL (about one and half years). I am a prod GNU Emacs user.

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