How to do multithrading of a simple function?

[heiner55]

#!/usr/bin/python3
import threading, time
from random import *

class cal():
    def __init__(self, i, a, b):
        self.i = i
        self.a = a
        self.b = b
    def add(self, c):
        print("thread", self.i, "starts", self.a, self.b, c)
        time.sleep(2)
        s = self.a + self.b + c
        print("thread", self.i, "ends: ", self.a, self.b, c, "sum: ", s)

for i in range(10):
    a = randint(1, 10)
    b = randint(1, 10)
    c = randint(1, 10)
    obj = cal(i, a, b)
    t = threading.Thread(target=obj.add, args=(c,))
    t.start()