Dice Class in Python

import random

class Dice:
    def __init__(self):
        self.__value = random.randint(1,6)

    def roll(self):
        self.__value = random.randint(1,6)

    def __str__(self):
        switcher = {1: "one", 2: "two", 3: "three", 4: "four",
                    5: "five", 6: "six"}
        return ("Next roll:  " + switcher[self.__value])


def main():
    dice1 = Dice()
    dice2 = Dice()
    print(dice1)
    print(dice2)
    dice1.roll()
    dice2.roll()
    print(dice1)
    print(dice2)
    # The following gives AttributeError: 'Dice' object has no attribute '__value'
    # underscores __ creates private attribute by name mangling
    if (dice1.__value == dice2.__value):
        print("DOUBLES")

main()