Internal create index helper.

        :Parameters:
          - `keys`: a list of tuples [(key, type), (key, type), ...]
          - `index_options`: a dict of index options.

def __create_index(self, keys, index_options):
        """Internal create index helper.

        :Parameters:
          - `keys`: a list of tuples [(key, type), (key, type), ...]
          - `index_options`: a dict of index options.
        """
        index_doc = helpers._index_document(keys)
        index = {"key": index_doc}
        collation = validate_collation_or_none(
            index_options.pop('collation', None))
        index.update(index_options)

        with self._socket_for_writes() as sock_info:
            if collation is not None:
                if sock_info.max_wire_version < 5:
                    raise ConfigurationError(
                        'Must be connected to MongoDB 3.4+ to use collations.')
                else:
                    index['collation'] = collation
            cmd = SON([('createIndexes', self.name), ('indexes', [index])])
            try:
                self._command(
                    sock_info, cmd, read_preference=ReadPreference.PRIMARY,
                    codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,
                    write_concern=self.write_concern,
                    parse_write_concern_error=True)
            except OperationFailure as exc:
                if exc.code in common.COMMAND_NOT_FOUND_CODES:
                    index["ns"] = self.__full_name
                    wcn = (self.write_concern if
                           self.write_concern.acknowledged else WriteConcern())
                    self.__database.system.indexes._insert(
                        sock_info, index, True, False, False, wcn)
                else:
                    raise