def print_report(self, cr, uid, ids, context=None):
    active_id = context.get('active_id', [])
    datas = {'ids' : [active_id]}
    return {
        'type': 'ir.actions.report.xml',
        'report_name': 'pos.receipt', # mention your report name here
        'datas': datas,
    }

<button name="generate_report" type="object" string="Generate report"/>

@api.multi
def generate_report(self):

    return {
               'type': 'ir.actions.report.xml',
               'report_name': 'myModule.my_report'
           }