Why this code increment twice my fields

Sequence can be generated.the number is appearing automatically But it is generated at create new record and then it is also changed while saving the record. So sequence is generated as doubled as follows: C00002, C00004, C00006,..... How can I solve this sequence generated by one by one? My .py file: from osv import fields, osv class res_partner(osv.osv): _inherit = 'res.partner' _name = 'res.partner' _columns = { 'n_client' : fields.char('Client Number', size=64, readonly=True), 'n_supplier' : fields.char('Supplier Number', size=64, readonly=True), } _sql_constraints = [ ('name_uniq_1', 'unique(n_client)', 'Number of client must be unique!'), ('n_supplier_uniq', 'unique(n_supplier)', 'Number of supplier must be unique!'), ] _defaults = { 'n_client': lambda obj, cr, uid, context: obj.pool.get('ir.sequence').get(cr, uid, 'res.partner.customer'), 'n_supplier': lambda obj, cr, uid, context: obj.pool.get('ir.sequence').get(cr, uid, 'res.partner.supplier'), } my Xml file: number_client_sequenceres.partner.customernumber_client_sequenceres.partner.customerC5number_supplier_sequenceres.partner.suppliernumber_supplier_sequenceres.partner.supplierS5res.partner.form.inheritres.partner