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

a.introduction()
b.introduction()