Here is a sample Action chains python action that does a search, selects a random book id, then "view"s the EPUB format of that book.
Code:
def run(gui, settings, chain):
db = gui.current_db
ids = list(db.new_api.search('#mybool:true and formats:epub'))
if ids:
import random
random_id = random.choice(ids)
view_action = gui.iactions['View']
view_action.view_format_by_id(random_id, 'EPUB', open_at=None)