def promptforfloat(default=None, outstring="Value"):
    """
    Prompt for float values, returning NoneType for bogus results.
    Default value on <ENTER> key is any previously set value.
    """
    try:
        retval = float(input("Enter %s [%s]:  " % \
            (str(outstring), str(default))))
    except:
        retval = default
    return(retval)