Product package layout

Following Zope, Plone and AT's conventions, the content of our example product pakage will look like this:

- __init__.py
- configure.zcml
- config.py
- interfaces.py
- content
    - __init__.py
    - message.py
- profiles
    - default
- browser
    - __init__.py
    - configure.zcml
    - instantmessage.pt
- tests
    - __init__.py
    - base.py
    - test_setup.py

What is the purpose of these files and directories?

• __init__.py: The usual "Python package" initialization module;
• configure.zcml: Using Zope's new Configuration Markup Language (ZCML), this file configures the services or behaviour the Zope server needs to load at startup;
  
• config.py: Provides configuration variables for the product;
• interfaces.py: Where you define interfaces describing what the packages' classes will do;
• content: Contains the modules providing the implementation of the content types. In this case, it contains the message.py file where the 'InstantMessage' class should be defined;
  
• profiles/default: Contains a set of XML files that are needed to provide the settings that will be used by Plone's Quick-Installer tool when installing the product within Plone; this is what we call an Extension Profile, an artifact of Zope CMF's GenericSetup technology. Note that this replaces the old way of doing based on the Extensions/Install. More precisely, since Plone 3.0, you do not need that old-style technique;
  
• browser: The sub-package where the developer can add specific presentation code such as browser views and templates; the contained configure.zcml is used to provide these components registration.
  
• tests: Contains the unit tests code for the product.
  

If you have ZopeSkel installed, you can use the following command to create a similar structure:

paster create -t archetype example.archetype

Now we will go through the files one by one and add what we need to produce our application.