I use in my module "account__pl":

<report
            id="account_invoices__pl"
            model="account.invoice"
            string="Invoices PL"
            report_type="qweb-pdf"
            name="account__pl.report_invoice_pl"
            file="account__pl.report_invoice_pl"
            attachment_use="True"
            attachment="(object.state in ('open','paid')) and ('IPL'+(object.number or '').replace('/','')+'.pdf')"
            multi="True"
        />

and in report_invoice_pl.py:

class report_invoice_pl(osv.AbstractModel):
    _name = 'report.account.report_invoice'
    _inherit = 'report.abstract_report'
    _template = 'account__pl.report_invoice_pl'
    _wrapped_report_class = parse_invoice_pl 

or, in new style reports:

class report_invoice(models.AbstractModel):
    _name = 'report.account.report_invoice'
    _template = 'account__pl.report_invoice_pl'
    
    @api.multi
    def render_html(self, data=None):
                                                                     
        report_obj = self.env['report']
        report = report_obj._get_report_from_name(self._template)        
       
        docargs = {
            'doc_ids': self._ids,
            'doc_model': report.model,
            'docs': self,
        }

        return report_obj.render(self._template, docargs)

class report_invoice_pl(report_invoice):
    _name = 'report.account__pl.report_invoice_pl'

and in report_invoice_pl.py:

class report_invoice_pl(osv.AbstractModel):
    _name = 'report.account.report_invoice'
    _inherit = 'report.abstract_report'
    _template = 'account__pl.report_invoice_pl'
    _wrapped_report_class = parse_invoice_pl 

This Model used to embed old style reports