def _float_conv(string):
    """
    Convenience tool to convert from string to float.

    If empty string return None rather than an error.

    >>> _float_conv('12')
    12.0
    >>> _float_conv('')
    >>> _float_conv('12.324')
    12.324
    """
    try:
        floatstring = float(string)
    except Exception:
        floatstring = None
    return floatstring