salt.py

class SaltWater:
    @classmethod
    def make(cls, density, weight):
        s = weight * density
        w = weight - s
        return SaltWater(s, w)

    def __init__(self, s, w):
        self.w = w
        self.s = s

    def __repr__(self):
        return f'{self.density * 100:.01f}%, {self.weight:.0f}g'

    @property
    def density(self):
        return self.s / self.weight

    @property
    def weight(self):
        return self.s + self.w

    def addWater(self, w):
        self.w += w

    def addSalt(self, s):
        self.s += s

    def addSaltWater(self, sw):
        if not isinstance(sw, SaltWater):
            raise ValueError()
        self.s += sw.s
        self.w += sw.w

    def __add__(self, sw):
        if not isinstance(sw, SaltWater):
            raise ValueError()
        s = self.s + sw.s
        w = self.w + sw.w
        return SaltWater(s, w)

    def __radd__(self, sw):
        return self.__ad__(sw)


a = SaltWater.make(0.1, 400)
b = SaltWater.make(.16, 800)
print(a)
print(b)
c = a + b
print(c)