class Calculator: def __init__(self): self.result = 0 def __validate(self, num): if not isinstance(num, (int, float)): return False return True def add(self, num): if self.__validate(num): self.result += num else: print("Invalid number") calc = Calculator() calc.add(10) calc.add(5) print(calc.result) # calc.__validate(5) # This would cause an error