@webapi_check_local_site
    @webapi_login_required
    @webapi_response_errors(DOES_NOT_EXIST, INVALID_FORM_DATA,
                            PERMISSION_DENIED, NOT_LOGGED_IN)
    @webapi_request_fields(
        required=dict({
            'file_attachment_id': {
                'type': int,
                'description': 'The ID of the file attachment being '
                               'commented on.',
            },
        }, **BaseFileAttachmentCommentResource.REQUIRED_CREATE_FIELDS),
        optional=dict({
            'diff_against_file_attachment_id': {
                'type': int,
                'description': 'The ID of the file attachment that '
                               'file_attachment_id is diffed. The comment '
                               'applies to the diff between these two '
                               'attachments.',
                'added_in': '2.0',
            },
        }, **BaseFileAttachmentCommentResource.OPTIONAL_CREATE_FIELDS),
        allow_unknown=True
    )
    def create(self, request, file_attachment_id=None,
               diff_against_file_attachment_id=None, *args, **kwargs):
        """Creates a file comment on a review.

        This will create a new comment on a file as part of a review.
        The comment contains text only.
        """
        try:
            review_request = \
                resources.review_request.get_object(request, *args, **kwargs)
            review = resources.review.get_object(request, *args, **kwargs)
        except ObjectDoesNotExist:
            return DOES_NOT_EXIST

        if not resources.review.has_modify_permissions(request, review):
            return self.get_no_access_error(request)

        try:
            file_attachment = \
                FileAttachment.objects.get(pk=file_attachment_id,
                                           review_request=review_request)
        except ObjectDoesNotExist:
            return INVALID_FORM_DATA, {
                'fields': {
                    'file_attachment_id': ['This is not a valid file '
                                           'attachment ID'],
                }
            }

        diff_against_file_attachment = None

        if diff_against_file_attachment_id:
            try:
                diff_against_file_attachment = FileAttachment.objects.get(
                    pk=diff_against_file_attachment_id,
                    review_request=review_request)
            except __HOLE__:
                return INVALID_FORM_DATA, {
                    'fields': {
                        'diff_against_file_attachment_id': [
                            'This is not a valid file attachment ID'
                        ],
                    }
                }

        new_comment = self.create_comment(
            review=review,
            file_attachment=file_attachment,
            diff_against_file_attachment=diff_against_file_attachment,
            fields=('file_attachment', 'diff_against_file_attachment'),
            **kwargs)
        review.file_attachment_comments.add(new_comment)

        return 201, {
            self.item_result_key: new_comment,
        }
dataset/ETHPy150Open reviewboard/reviewboard/reviewboard/webapi/resources/review_file_attachment_comment.py/ReviewFileAttachmentCommentResource.create