def simple2string(x):
    """
    Simple objects (bytes, bool, float, int, None, str) are converted
    to string and returned. Other types are returned as None.
    """
    if  isinstance(x, bytes) or \
        isinstance(x, bool ) or \
        isinstance(x, float) or \
        isinstance(x, int  ) or \
        x is None            or \
        isinstance(x, str  ):
        return str(x)
    else:
        return None