I am using openerp 7 api for my development.I want to inherit report of partner ledger.The function is not executing, I think the syntax is correct but I not getting where I have done mistake.

from openerp.osv import fields,osv
class report_partnerledger(osv.AbstractModel):
    _inherit = 'report.account.report_partnerledger'

    def _check_status(self, cr, uid, ids, context=None):
        print "Entering check status func"
        iscredit = self.cr.execute("SELECT l.credit FROM account_move_line AS l")
        if(iscredit == 0):
            docm="Invoice"
        else:
            docm="Refunded Invoice"
        print docm
        return docm

    _columns = {
        'doc_info': fields.function(_check_status,type='char'),
    }