Return s as a bytes type, using utf-8 encoding if necessary.
    @param s string or bytes
    @return bytes

def to_bytes(s):
    '''Return s as a bytes type, using utf-8 encoding if necessary.
    @param s string or bytes
    @return bytes
    '''
    if isinstance(s, six.binary_type):
        return s
    if isinstance(s, six.string_types):
        return s.encode('utf-8')
    raise TypeError('want string or bytes, got {}', type(s))