def get_attr_value(self, attr_key, el_idx=0):
        """Return the value of the selected attribute in the selected element.

        Args:
          attr_key : str
            Name of attribute for which to search

          el_idx : int
            Index of element to use in the event that there are multiple sibling
            elements with the same name.

        Returns:
          str : Value of the selected attribute in the selected element.

        """
        return self.get_element_by_attr_key(attr_key, el_idx).attrib[attr_key]