How to use the nbgrader.utils.is_locked function in nbgrader

def _get_passed_cells(self, nb: NotebookNode) -> typing.List[NotebookNode]:
        passed = []
        for cell in nb.cells:
            if not (utils.is_grade(cell) or utils.is_locked(cell)):
                continue

            # if it's a grade cell, the check the grade
            if utils.is_grade(cell):
                score, max_score = utils.determine_grade(cell, self.log)

                # it's a markdown cell, so we can't do anything
                if score is None:
                    pass
                elif score == max_score:
                    passed.append(cell)

        return passed

def preprocess_cell(self,
                        cell: NotebookNode,
                        resources: ResourcesDict,
                        cell_index: int
                        ) -> Tuple[NotebookNode, ResourcesDict]:
        if (self.lock_solution_cells or self.lock_grade_cells) and utils.is_solution(cell) and utils.is_grade(cell):
            cell.metadata['deletable'] = False
        elif self.lock_solution_cells and utils.is_solution(cell):
            cell.metadata['deletable'] = False
        elif self.lock_grade_cells and utils.is_grade(cell):
            cell.metadata['deletable'] = False
            cell.metadata['editable'] = False
        elif self.lock_readonly_cells and utils.is_locked(cell):
            cell.metadata['deletable'] = False
            cell.metadata['editable'] = False
        elif self.lock_all_cells:
            cell.metadata['deletable'] = False
            cell.metadata['editable'] = False
        return cell, resources

source_cell = self.gradebook.find_source_cell(
                grade_id,
                self.notebook_id,
                self.assignment_id)
        except MissingEntry:
            self.log.warning("Cell '{}' does not exist in the database".format(grade_id))
            del cell.metadata.nbgrader['grade_id']
            return cell, resources

        # check that the cell type hasn't changed
        if cell.cell_type != source_cell.cell_type:
            self.report_change(grade_id, "cell_type", source_cell.cell_type, cell.cell_type)
            self.update_cell_type(cell, source_cell.cell_type)

        # check that the locked status hasn't changed
        if utils.is_locked(cell) != source_cell.locked:
            self.report_change(grade_id, "locked", source_cell.locked, utils.is_locked(cell))
            cell.metadata.nbgrader["locked"] = source_cell.locked

        # if it's a grade cell, check that the max score hasn't changed
        if utils.is_grade(cell):
            grade_cell = self.gradebook.find_grade_cell(
                grade_id,
                self.notebook_id,
                self.assignment_id)
            old_points = float(grade_cell.max_score)
            new_points = float(cell.metadata.nbgrader["points"])

            if old_points != new_points:
                self.report_change(grade_id, "points", old_points, new_points)
                cell.metadata.nbgrader["points"] = old_points

grade_id,
                self.notebook_id,
                self.assignment_id)
        except MissingEntry:
            self.log.warning("Cell '{}' does not exist in the database".format(grade_id))
            del cell.metadata.nbgrader['grade_id']
            return cell, resources

        # check that the cell type hasn't changed
        if cell.cell_type != source_cell.cell_type:
            self.report_change(grade_id, "cell_type", source_cell.cell_type, cell.cell_type)
            self.update_cell_type(cell, source_cell.cell_type)

        # check that the locked status hasn't changed
        if utils.is_locked(cell) != source_cell.locked:
            self.report_change(grade_id, "locked", source_cell.locked, utils.is_locked(cell))
            cell.metadata.nbgrader["locked"] = source_cell.locked

        # if it's a grade cell, check that the max score hasn't changed
        if utils.is_grade(cell):
            grade_cell = self.gradebook.find_grade_cell(
                grade_id,
                self.notebook_id,
                self.assignment_id)
            old_points = float(grade_cell.max_score)
            new_points = float(cell.metadata.nbgrader["points"])

            if old_points != new_points:
                self.report_change(grade_id, "points", old_points, new_points)
                cell.metadata.nbgrader["points"] = old_points

        # always update the checksum, just in case