import six


def to_float(x):
    """Convert an object to a float, unless it is None
    
    Examples:
    >>> print(to_float(12.3))
    12.3
    >>> print(to_float('12.3'))
    12.3
    >>> print(to_float(None))
    None
    """
    if isinstance(x, six.string_types) and x.lower() == 'none':
        x = None
    return float(x) if x is not None else None