Python Classes¶

You'll have noticed that Python classes are often part of the wiring of Components, and you will find that you can't really avoid understanding a little bit about them, particularly if you want to make your own viewlets.

Having to deal with something as advanced as Python classes can be daunting for the non-developer. The good news is that using Python classes will be more a case of copying and changing little bits of code than writing anything from scratch.

What's a Class?¶

It's best to think of a class as a discrete piece of code containing a collection of methods ('actions' of some sort) and attributes ('variables' which can hold a value).

In the case of components, the main purpose of a class is to compute the pieces of information a component needs to display. The class for the logo viewlet is a good example. You can find it in:

[your egg location]/plone/app/layout/viewlets/common.py - look for LogoViewlet

After a bit of preparatory work, the LogoViewlet class first finds out the name of the image that is to be used for the logo (and is defined in the base_properties property sheet):

                        logoName = portal.restrictedTraverse('base_properties').logoName

                      

Then it works out the logo's vital statistics, size, alt text etc and turns this into an HTML anchor tag:

                        self.logo_tag = portal.restrictedTraverse(logoName).tag()

                      

Finally, just in case you might need it, it looks up the title of the site:

                        self.portal_title = self.portal_state.portal_title()

                      

In the page template associated with this viewlet you can get hold of this information (self.logo_tag, self.portal_title) using the variable "view":

                        <img src="logo.jpg" alt=""
         tal:replace="structure view/logo_tag" />

Do I have to use Classes?¶

Viewlets tend to be wired up with a Python class which points to a template. So, even though you might only want to create a new template, you'll find that you have to write a class to point to your new template. The Elements section of this manual should help you by giving you a snippet of code for each element to copy and paste into your own product.

Here's an example. The standard logo template doesn't actually make use of view/portal_title. So if you wanted to incorporate this into your logo in some way, then you would need to write your own template and then also your own class:

                        from plone.app.layout.viewlets.common import LogoViewlet
from Products.Five.browser.pagetemplatefile import ViewPageTemplateFile

class [your class name](LogoViewlet):
    render = ViewPageTemplateFile('[your template name]')

First, pull in ("import") all the bits and pieces with which to build your class using from ….. import …… .
Next, define your class. The important thing here is to base it on a pre-existing class so that you don't have to start from scratch. Put the name of the pre-existing class in brackets after your class name (make sure that you've imported it first). Don't forget the colon!
Finally, rewrite any of the methods or attributes you need. Here, we've just rewritten the render method to display our own template.

Note: indenting is very important in Python code, the convention is to use four spaces (rather than a tab). If you are having problems, double check the indentation first.

http://wiki.python.org/moin/HowToEditPythonCode

If you're feeling brave or want to know more, a straightforward introduction is here: http://www.diveintopython.org/object_oriented_framework/defining_classes.html

Dive Into Python - Defining Classes