code: def get_attrib(xml, name, tag=None, cast=str, default=None):
      
      if name in xml.attrib:
          return cast(xml.attrib[name])
      elif default is not None:
          return default
      elif tag is not None:
          raise ValueError("'{}' is a required attribute of <{}> tag.".format(name, tag))
docs: Returns the specified attribute from the XML element, raising a ValueError
      if it is not availaible.
      
      :arg xml: the XMLElement instance to get the attribute from.
      :arg name: the name of the attribute in xml.attrib dictionary.
      :arg tag: the name of the tag to display with the ValueError if the attribute is missing.