[Boilerplate4U]

Could it be this one?
https://github.com/kovidgoyal/calibre/blob/master/src/calibre/ebooks/metadata/opf2.py

My guess the none match in the xpath search "isbn_path" is caused by the "and" clause as in: "and (re:match(@scheme, "isbn", "i") or re:match(@opf:scheme, "isbn", "i"))]) marked in red below:

class OPF(object):  # {{{

    MIMETYPE         = 'application/oebps-package+xml'
    NAMESPACES       = {
                        None: "http://www.idpf.org/2007/opf",
                        'dc': "http://purl.org/dc/elements/1.1/",
                        'opf': "http://www.idpf.org/2007/opf",
                       }
    META             = '{%s}meta' % NAMESPACES['opf']
    xpn = NAMESPACES.copy()
    xpn.pop(None)
    xpn['re'] = 'http://exslt.org/regular-expressions'
    XPath = functools.partial(etree.XPath, namespaces=xpn)
    CONTENT          = XPath('self::*[re:match(name(), "meta$", "i")]/@content')
    TEXT             = XPath('string()')

    metadata_path   = XPath('descendant::*[re:match(name(), "metadata", "i")]')
    metadata_elem_path = XPath(
        'descendant::*[re:match(name(), concat($name, "$"), "i") or (re:match(name(), "meta$", "i") '
        'and re:match(@name, concat("^calibre:", $name, "$"), "i"))]')
    title_path      = XPath('descendant::*[re:match(name(), "title", "i")]')
    authors_path    = XPath('descendant::*[re:match(name(), "creator", "i") and (@role="aut" or @opf:role="aut" or (not(@role) and not(@opf:role)))]')
    bkp_path        = XPath('descendant::*[re:match(name(), "contributor", "i") and (@role="bkp" or @opf:role="bkp")]')
    tags_path       = XPath('descendant::*[re:match(name(), "subject", "i")]')
    isbn_path       = XPath('descendant::*[re:match(name(), "identifier", "i") and '
                            '(re:match(@scheme, "isbn", "i") or re:match(@opf:scheme, "isbn", "i"))]')