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

def _clear_existing_files(self, path):
        if not os.listdir(path):
            return

        if self.force:
            self.log.warning("Clearing existing files in {}".format(path))
            rmtree(path)
            os.makedirs(path)
        else:
            self.fail(
                "Directory not empty: {}\nuse the --force option to clear "
                "previously existing files".format(path)
            )

path = os.path.join(dest, filename)
                    if os.path.exists(path):
                        self.log.warning("Removing existing notebook: {}".format(path))
                        remove(path)
            return True

        src = self._format_source(assignment_id, student_id)
        new_timestamp = self.coursedir.get_existing_timestamp(src)
        old_timestamp = self.coursedir.get_existing_timestamp(dest)

        # if --force hasn't been specified, but the source assignment is newer,
        # then we want to overwrite it
        if new_timestamp is not None and old_timestamp is not None and new_timestamp > old_timestamp:
            if self.coursedir.notebook_id == "*":
                self.log.warning("Updating existing assignment: {}".format(dest))
                rmtree(dest)
            else:
                for notebook in self.notebooks:
                    filename = os.path.splitext(os.path.basename(notebook))[0] + self.exporter.file_extension
                    path = os.path.join(dest, filename)
                    if os.path.exists(path):
                        self.log.warning("Updating existing notebook: {}".format(path))
                        remove(path)
            return True

        # otherwise, we should skip the assignment
        self.log.info("Skipping existing assignment: {}".format(dest))
        return False

def _handle_failure(gd):
            dest = os.path.normpath(self._format_dest(gd['assignment_id'], gd['student_id']))
            if self.notebook_id == "*":
                if os.path.exists(dest):
                    self.log.warning("Removing failed assignment: {}".format(dest))
                    rmtree(dest)
            else:
                for notebook in self.notebooks:
                    filename = os.path.splitext(os.path.basename(notebook))[0] + self.exporter.file_extension
                    path = os.path.join(dest, filename)
                    if os.path.exists(path):
                        self.log.warning("Removing failed notebook: {}".format(path))
                        remove(path)

def _handle_failure(gd: typing.Dict[str, str]) -> None:
            dest = os.path.normpath(self._format_dest(gd['assignment_id'], gd['student_id']))
            if self.coursedir.notebook_id == "*":
                if os.path.exists(dest):
                    self.log.warning("Removing failed assignment: {}".format(dest))
                    rmtree(dest)
            else:
                for notebook in self.notebooks:
                    filename = os.path.splitext(os.path.basename(notebook))[0] + self.exporter.file_extension
                    path = os.path.join(dest, filename)
                    if os.path.exists(path):
                        self.log.warning("Removing failed notebook: {}".format(path))
                        remove(path)

if self.coursedir.notebook_id == "*":
            if not os.path.exists(dest):
                return True
        else:
            # if any of the notebooks don't exist, then we want to process them
            for notebook in self.notebooks:
                filename = os.path.splitext(os.path.basename(notebook))[0] + self.exporter.file_extension
                path = os.path.join(dest, filename)
                if not os.path.exists(path):
                    return True

        # if we have specified --force, then always remove existing stuff
        if self.force:
            if self.coursedir.notebook_id == "*":
                self.log.warning("Removing existing assignment: {}".format(dest))
                rmtree(dest)
            else:
                for notebook in self.notebooks:
                    filename = os.path.splitext(os.path.basename(notebook))[0] + self.exporter.file_extension
                    path = os.path.join(dest, filename)
                    if os.path.exists(path):
                        self.log.warning("Removing existing notebook: {}".format(path))
                        remove(path)
            return True

        src = self._format_source(assignment_id, student_id)
        new_timestamp = self.coursedir.get_existing_timestamp(src)
        old_timestamp = self.coursedir.get_existing_timestamp(dest)

        # if --force hasn't been specified, but the source assignment is newer,
        # then we want to overwrite it
        if new_timestamp is not None and old_timestamp is not None and new_timestamp > old_timestamp:

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

        # make sure the course id was provided
        if len(self.extra_args) != 1:
            self.fail("Course id not provided. Usage: nbgrader quickstart course_id")

        # make sure it doesn't exist
        course_id = self.extra_args[0]
        course_path = os.path.abspath(course_id)
        if os.path.exists(course_path):
            if self.force:
                self.log.warning("Removing existing directory '%s'", course_path)
                utils.rmtree(course_path)
            else:
                self.fail(
                    "Directory '{}' already exists! Rerun with --force to remove "
                    "this directory first (warning: this will remove the ENTIRE "
                    "directory and all files in it.) ".format(course_path))

        # create the directory
        self.log.info("Creating directory '%s'...", course_path)
        os.mkdir(course_path)

        # populating it with an example
        self.log.info("Copying example from the user guide...")
        example = os.path.abspath(os.path.join(
            os.path.dirname(__file__), '..', 'docs', 'source', 'user_guide', 'source'))
        shutil.copytree(example, os.path.join(course_path, "source"))