Hi,

use ORM method browse.

Syntax :browse(cr, uid, select, context=None, list_class=None, fields_process=None)

Example :

def write(self, cr, uid, ids, vals, context=None):
    partner = self.browse(cr, uid, ids[0], context=context)
    #for example get first_name
    first_name=partner.first_name
    # all other field partner.field_name

Thank You.