@staticmethod
def validate_wunit(w):
    """
        @param w widget mapping
        """
    err = []
    try:
        v = w['symbol'].get()
        if len(v) < 1 or len(v) > 2:
            stat = False
            err.append(('symbol', 'char length is not valid'))
            w['symbol'].config(bg='red')
        else:
            w['symbol'].config(bg='white')
    except:
        err.append(('symbol', 'exception orruced'))
    for name in ['z', 'w', 'stoich', 'disp']:
        e = w[name].validate()
        if e:
            err.append((name, e))
    return err if err else None
