Hi,

you must define the method create in your custom module and make tests in this function.

def create(self, cr, uid, vals, context=None):
       if context is None:
            context={}       
       if  not vals.get('is_company') : 
            if not vals.get('first_name') :
                raise osv.except_osv(('Error first_name'), 
                                     (' first_name required  ') )

            if not vals.get('last_name') :
                raise osv.except_osv(('Error last_name'), 
                                     (' last_name required  ') )
      return super(res_partner,self).create(cr, uid, vals, context=context)

and same in methods write and copy.

Thanks.