Get the first descendant of source identified by path.

  Path must be either a an xpath string, or a 2-tuple of (xpath, namespace_dict).

def get_xml_child(source, path):
  """Get the first descendant of source identified by path.

  Path must be either a an xpath string, or a 2-tuple of (xpath, namespace_dict).
  """
  if isinstance(path, (tuple, list)):
    return source.find(*path)
  else:
    return source.find(path)