def close(self):
        """Closes the connection.
        No further operations are allowed, either on the connection or any
        of its cursors, once the connection is closed.

        If the connection is used in a ``with`` statement, this method will
        be automatically called at the end of the ``with`` block.
        """
        if self._closed:
            raise ProgrammingError('the connection is already closed')
        for cursor_ref in self._cursors:
            cursor = cursor_ref()
            if cursor is not None and not cursor._closed:
                cursor.close()
        self._client.close_connection(self._id)
        self._client.close()
        self._closed = True