In openerp If the report is using a custom model to render its html and fallback on the generic html rendering. report is generates and returns html version of a new report.
use this function in your own module in .py file,

    def get_html(self, cr, uid, ids, report_name, data=None, context=None):
        """This method generates and returns html version of a report.
        """
        # If the report is using a custom model to render its html, we must use it.
        # Otherwise, fallback on the generic html rendering.
        try:
            report_model_name = 'report.%s' % report_name
            particularrepor_obj = self.pool[report_model_name]
            return particularreport_obj.render_html(cr, uid, ids, data=data, context=context)
        except KeyError:
            report = self._get_report_from_name(cr, uid, report_name)
            report_obj = self.pool[report.model]
            docs = report_obj.browse(cr, uid, ids, context=context)
            docargs = {
                'doc_ids': ids,
                'doc_model': report.model,
                'docs': docs,
            }
            return self.render(cr, uid, [], report.report_name, docargs, context=context)