def _getElemAttrib(parentElem, elemName, elemAttribName):
    """
    XML parsing helper function to find child element with a specific name, and return the value of a specified attribute.
    
    :param SubElement parentElem: the :py:class:`XML SubElement <xml.etree.ElementTree.SubElement>` to query.
    :param str elemName: search for XML elements with this tag.
    :param str elemAttribName: filter on those XML elements with this attribute name.
    
    :returns: the attribute value associated with the XML element whose tag is ``elemName`` and whose attribute is ``elemAttribName``.
    :rtype: str
    """
    elem = parentElem.find(elemName)
    if elem is not None:
        return elem.attrib[elemAttribName]
    return None