main.py

class FirstTest:
    def __init__(self, arg1, arg2):
        '''Try to define vars'''
        self.arg1 = arg1
        self.arg2 = arg2

    def test(self):
        message = (self.arg1 + " - this is arg1!")

        message2 = (self.arg2 + " - this is arg2")

        print("FirstTest class in runnging: " + message + " and " + message2)


class SecondTest(FirstTest):
    def __init__(self, arg1, arg2, arg3):
        '''See on new arg3 parametr, I, add him in this new class, what was extended from parent FirstClass'''
        super().__init__(arg1, arg2)
        self.arg3 = arg3

    def test2(self):
        message3 = self.arg3

        finalmsg = "New SecondTest class calling arg1: {argm1}, arg2: {argm2} from " \
                   "FirstClass from method test and arg3: {argm3} " \
                   "from SecondTest class , test2() method".format(argm1=self.arg1, argm2=self.arg2, argm3=message3)

        print(finalmsg)