def getCanonicalID(iname, xrd_tree):
    """Return the CanonicalID from this XRDS document.

    @param iname: the XRI being resolved.
    @type iname: unicode

    @param xrd_tree: The XRDS output from the resolver.
    @type xrd_tree: ElementTree

    @returns: The XRI CanonicalID or None.
    @returntype: unicode or None
    """
    xrd_list = xrd_tree.findall(xrd_tag)
    xrd_list.reverse()

    try:
        canonicalID = xri.XRI(xrd_list[0].findall(canonicalID_tag)[0].text)
    except __HOLE__:
        return None

    childID = canonicalID.lower()

    for xrd in xrd_list[1:]:
        parent_sought = childID.rsplit("!", 1)[0]
        parent = xri.XRI(xrd.findtext(canonicalID_tag))
        if parent_sought != parent.lower():
            raise XRDSFraud("%r can not come from %s" % (childID, parent))

        childID = parent_sought

    root = xri.rootAuthority(iname)
    if not xri.providerIsAuthoritative(root, childID):
        raise XRDSFraud("%r can not come from root %r" % (childID, root))

    return canonicalID
dataset/ETHPy150Open necaris/python3-openid/openid/yadis/etxrd.py/getCanonicalID