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/ - 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 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.

If you're feeling brave or want to know more, a straightforward introduction is here: