code: def _get_placeholder_arg(arg_name, placeholder):
      
      if placeholder is None:
          raise RuntimeWarning(u"placeholder object is None")
      elif isinstance(placeholder, Placeholder):
          return placeholder
      elif isinstance(placeholder, Manager):
          manager = placeholder
          try:
              parent_object = manager.instance  # read RelatedManager code
          except AttributeError:
              parent_object = None
  
          try:
              placeholder = manager.all()[0]
              if parent_object is not None:
                  placeholder.parent = parent_object  # Fill GFK cache
              return placeholder
          except IndexError:
              raise RuntimeWarning(u"No placeholders found for query '{0}.all.0'".format(arg_name))
      else:
          raise ValueError(u"The field '{0}' does not refer to a placeholder object!".format(arg_name))
docs: Validate and return the Placeholder object that the template variable points to.