code: def prepare(self, context):
          
          self.params = {}
  
          # no params, not even a newline
          if not self.nodelist:
              return
  
          # just static text, no vars, assume one TextNode
          if not self.nodelist.contains_nontext:
              text = self.nodelist[0].s.strip()
  
          # vars in params, we have to render
          else:
              context.push()
              context['object'] = self.obj
              text = self.nodelist.render(context)
              context.pop()
  
          if text:
              self.params = self.resolve_params(text)
  
          # override the default template from the parameters
          if 'template_name' in self.params:
              self.template_name = self.params['template_name']
docs: Do the pre-processing - render and parse the parameters and
          store them for further use in self.params.