Use of @property decorator

[Yoriz]

Another way, keep track of the previous values, only re calculate if they have changed.

class Vessel():
    def __init__(self, class_notation, length):
        self.class_notation = class_notation
        self.length = length
        self.previous_values = (None, None, None)

    @property
    def wave_coef(self):
        if self.previous_values[:2] == (self.class_notation, self.length):
            return self.previous_values[2]
        print('calculating wave_coef')
        table = {"R0": 1,
                 "R1": 0.9,
                 "R2": 0.8,
                 "R3": 0.7,
                 "R4": 0.6,
                 "R5": 0.5,
                 "R6": 0.4}
        service_factor = table[self.class_notation]

        if (self.length <= 100):
            CW = 0.08 * self.length * service_factor
        else:
            CW = 6 + 0.02 * self.length * service_factor

        self.previous_values = (self.class_notation, self.length, CW)
        return CW


vessel = Vessel('R2', 2)
print(vessel.wave_coef)
print(vessel.wave_coef)
vessel.class_notation = 'R4'
print(vessel.wave_coef)
vessel.length = 1
print(vessel.wave_coef)

Output:
calculating wave_coef
0.128
0.128
calculating wave_coef
0.096
calculating wave_coef
0.048