Reimplements the :meth:`QAbstractItemModel.index` method.

        :param row: Row.
        :type row: int
        :param column: Column.
        :type column: int
        :param parent: Parent.
        :type parent: QModelIndex
        :return: Index.
        :rtype: QModelIndex

def index(self, row, column=0, parent=QModelIndex()):
        """
        Reimplements the :meth:`QAbstractItemModel.index` method.

        :param row: Row.
        :type row: int
        :param column: Column.
        :type column: int
        :param parent: Parent.
        :type parent: QModelIndex
        :return: Index.
        :rtype: QModelIndex
        """

        parent_node = self.get_node(parent)
        child = parent_node.child(row)
        if child:
            return self.createIndex(row, column, child)
        else:
            return QModelIndex()