def validate_float_or_None(s):
    """convert s to float, None or raise"""
    # values directly from the rc file can only be strings,
    # so we need to recognize the string "None" and convert
    # it into the object. We will be case-sensitive here to
    # avoid confusion between string values of 'none', which
    # can be a valid string value for some other parameters.
    if s is None or s == 'None':
        return None
    try:
        return float(s)
    except ValueError:
        raise ValueError('Could not convert "%s" to float or None' % s)