temperature conversion function

conversions = {
            'Celsius': (1, 0),
            'Farenheit': (1.8, 32),
            'Kelvin': (1, 273.15)
        }
def convert_temp(temp, from_unit, to_unit):
    try:
        divide, subtract = conversions[from_unit]
        multiply, add = conversions[to_unit]
    except KeyError:
        raise KeyError('Unknown unit %s, %s' % from_unit, to_unit)
    return (temp - subtract) * multiply / divide + add