def _partialmatch(self, id):
        if id in self._pcache:
            return self._pcache[id]

        if len(id) < 40:
            try:
                # hex(node)[:...]
                l = len(id) // 2  # grab an even number of digits
                prefix = bin(id[:l * 2])
                nl = [e[7] for e in self.index if e[7].startswith(prefix)]
                nl = [n for n in nl if hex(n).startswith(id)]
                if len(nl) > 0:
                    if len(nl) == 1:
                        self._pcache[id] = nl[0]
                        return nl[0]
                    raise LookupError(id, self.indexfile,
                                      _('ambiguous identifier'))
                return None
            except TypeError:
                pass