How to use the nbgrader.apps.NbGrader function in nbgrader

def start(self):
        super(DbStudentListApp, self).start()

        with Gradebook(self.coursedir.db_url) as gb:
            print("There are %d students in the database:" % len(gb.students))
            for student in gb.students:
                print("%s (%s, %s) -- %s" % (student.id, student.last_name, student.first_name, student.email))


assignment_add_aliases = {}
assignment_add_aliases.update(aliases)
assignment_add_aliases.update({
    'duedate': 'DbAssignmentAddApp.duedate',
})

class DbAssignmentAddApp(NbGrader):

    name = 'nbgrader-db-assignment-add'
    description = 'Add an assignment to the nbgrader database'

    aliases = assignment_add_aliases
    flags = flags

    duedate = Unicode(
        None,
        allow_none=True,
        help="The due date of the assignment"
    ).tag(config=True)

    def start(self):
        super(DbAssignmentAddApp, self).start()

# and that any empty strings are parsed as None
                    assignment = {}
                    for key, val in row.items():
                        if key not in allowed_keys:
                            continue
                        if val == '':
                            assignment[key] = None
                        else:
                            assignment[key] = val
                    assignment_id = assignment.pop("name")

                    self.log.info("Creating/updating assignment with name '%s': %s", assignment_id, assignment)
                    gb.update_or_create_assignment(assignment_id, **assignment)


class DbAssignmentListApp(NbGrader):

    name = 'nbgrader-db-assignment-list'
    description = 'List assignments int the nbgrader database'

    aliases = aliases
    flags = flags

    def start(self):
        super(DbAssignmentListApp, self).start()

        with Gradebook(self.coursedir.db_url) as gb:
            print("There are %d assignments in the database:" % len(gb.assignments))
            for assignment in gb.assignments:
                print("%s (due: %s)" % (assignment.name, assignment.duedate))
                for notebook in assignment.notebooks:
                    print("    - %s" % notebook.name)

def load_config(self):
        paths = jupyter_config_path()
        paths.insert(0, os.getcwd())

        app = NbGrader()
        app.config_file_paths.append(paths)
        app.load_config_file()

        return app.config

def load_config(self):
        paths = jupyter_config_path()
        paths.insert(0, os.getcwd())

        app = NbGrader()
        app.config_file_paths.append(paths)
        app.load_config_file()

        return app.config

aliases = aliases
    flags = flags

    def start(self):
        super(DbAssignmentListApp, self).start()

        with Gradebook(self.coursedir.db_url) as gb:
            print("There are %d assignments in the database:" % len(gb.assignments))
            for assignment in gb.assignments:
                print("%s (due: %s)" % (assignment.name, assignment.duedate))
                for notebook in assignment.notebooks:
                    print("    - %s" % notebook.name)


class DbStudentApp(NbGrader):

    name = 'nbgrader-db-student'
    description = u'Modify or list students in the nbgrader database'

    subcommands = {
        "add": (DbStudentAddApp, "Add a student to the database"),
        "remove": (DbStudentRemoveApp, "Remove a student from the database"),
        "list": (DbStudentListApp, "List students in the database"),
        "import": (DbStudentImportApp, "Import students into the database from a file")
    }

    @default("classes")
    def _classes_default(self):
        classes = super(DbStudentApp, self)._classes_default()

        # include all the apps that have configurable options

}

        self.log.info("Creating/updating student with ID '%s': %s", student_id, student)
        with Gradebook(self.coursedir.db_url) as gb:
            gb.update_or_create_student(student_id, **student)

student_remove_flags = {}
student_remove_flags.update(flags)
student_remove_flags.update({
    'force': (
        {'DbStudentRemoveApp': {'force': True}},
        "Complete the operation, even if it means grades will be deleted."
    ),
})

class DbStudentRemoveApp(NbGrader):

    name = 'nbgrader-db-student-remove'
    description = 'Remove a student from the nbgrader database'

    aliases = aliases
    flags = student_remove_flags

    force = Bool(False, help="Confirm operation if it means grades will be deleted.").tag(config=True)

    def start(self):
        super(DbStudentRemoveApp, self).start()

        if len(self.extra_args) != 1:
            self.fail("No student id provided.")

        student_id = self.extra_args[0]

except MissingEntry:
                self.fail("No such assignment: '%s'", assignment_id)

            if len(assignment.submissions) > 0:
                if self.force:
                    self.log.warning("Removing associated grades")
                else:
                    self.log.warning("!!! There are grades in the database for assignment '%s'.", assignment_id)
                    self.log.warning("!!! Removing this assignment will also remove these grades.")
                    self.fail("!!! If you are SURE this is what you want to do, rerun with --force.")

            self.log.info("Removing assignment with ID '%s'", assignment_id)
            gb.remove_assignment(assignment_id)


class DbAssignmentImportApp(NbGrader):

    name = 'nbgrader-db-assignment-import'
    description = 'Import assignments into the nbgrader database from a CSV file'

    aliases = aliases
    flags = flags

    def start(self):
        super(DbAssignmentImportApp, self).start()

        if len(self.extra_args) != 1:
            self.fail("Path to CSV file not provided.")

        path = self.extra_args[0]
        if not os.path.exists(path):
            self.fail("No such file: '%s'", path)

self.log.info("Creating/updating assignment with ID '%s': %s", assignment_id, assignment)
        with Gradebook(self.coursedir.db_url) as gb:
            gb.update_or_create_assignment(assignment_id, **assignment)


assignment_remove_flags = {}
assignment_remove_flags.update(flags)
assignment_remove_flags.update({
    'force': (
        {'DbAssignmentRemoveApp': {'force': True}},
        "Complete the operation, even if it means grades will be deleted."
    ),
})

class DbAssignmentRemoveApp(NbGrader):

    name = 'nbgrader-db-assignment-remove'
    description = 'Remove an assignment from the nbgrader database'

    aliases = aliases
    flags = assignment_remove_flags

    force = Bool(False, help="Confirm operation if it means grades will be deleted.").tag(config=True)

    def start(self):
        super(DbAssignmentRemoveApp, self).start()

        if len(self.extra_args) != 1:
            self.fail("No assignment id provided.")

        assignment_id = self.extra_args[0]

except MissingEntry:
                self.fail("No such student: '%s'", student_id)

            if len(student.submissions) > 0:
                if self.force:
                    self.log.warning("Removing associated grades")
                else:
                    self.log.warning("!!! There are grades in the database for student '%s'.", student_id)
                    self.log.warning("!!! Removing this student will also remove these grades.")
                    self.fail("!!! If you are SURE this is what you want to do, rerun with --force.")

            self.log.info("Removing student with ID '%s'", student_id)
            gb.remove_student(student_id)


class DbStudentImportApp(NbGrader):

    name = 'nbgrader-db-student-import'
    description = 'Import students into the nbgrader database from a CSV file'

    aliases = aliases
    flags = flags

    def start(self):
        super(DbStudentImportApp, self).start()

        if len(self.extra_args) != 1:
            self.fail("Path to CSV file not provided.")

        path = self.extra_args[0]
        if not os.path.exists(path):
            self.fail("No such file: '%s'", path)