import numpy as np

class Ball:
    __diameter__ = 0.0
    
    ## Methods
    # Constructor
    def __init__(self, diameter = 1.0):
        self.__diameter__ = diameter
        pass
        
    # a simple method
    def introduction(self):
        print(f"My diameter is {self.__diameter__:.3f}.")
        pass

    # Getter
    def get_diameter(self):
        return self.__diameter__

    # overload __add_ (+)
    def __add__(self, other):
        d1 = self.get_diameter()

        if(isinstance(other, Ball)):
            d2 = other.get_diameter()
            combined_volume = 1/6 * np.pi * (d1**3 + d2**3)
        elif(isinstance(other, int) or isinstance(other, float)):
            combined_volume = 1/6 * np.pi*d1**3 + other
        else:
            raise ValueError("Invalid summand.")
        d_combined = np.power(6 * combined_volume / np.pi, 1/3)
        return Ball(d_combined)
        
    # overload __radd__ (number + Ball)
    def __radd__(self, other):
        return self + other

# main code
a = Ball()
b = Ball(2.0)

c = a + b

d = a + 5
e = 5 + a

a.introduction()
b.introduction()
c.introduction()
d.introduction()