6.3.3 Helpful Hints for Developing Custom Tags

Your custom tag module is imported when PSE itself is initialized, and is therefore a long-living module. Keep in mind that any modules imported globally will only be initialized once for each Apache process.
The pse module (and all plugins) is only available when a servlet is running, so you cannot import it globally. Import pse and any plugins you want to access inside your custom tag methods.
The easiest way to get all the content between your custom tag is to call CustomTag.__str__(self) in your __str__ method. You can also manually loop through the self._contents list if you want to do something specific for each bit that was added.
Don't forget that you can always override the methods add and addString to do what you want. For example, if you wanted to ignore nested custom tags, just do pass for add. Or, keep the bare HTML as a separate list in addString. You can always make a super call to add functionality while retaining the default actions of the CustomTag class.
As an instructional aid, you may want to look at the .pty file that PSE generates to see how custom tags are converted to Python code.