def stn ( s , length , encoding , errors ) : s = s . encode ( encoding , errors ) return s [ : length ] + ( length - len ( s ) ) * NUL
how do you use nbytes in python
Convert a string to a null - terminated bytes object .
def stn(s, length, encoding, errors):
    """Convert a string to a null-terminated bytes object.
    """
    s = s.encode(encoding, errors)
    return s[:length] + (length - len(s)) * NUL