Straightforward programming:

def add_two_numbers(a, b):
    return a + b

print(add_two_numbers(4, 5)) #prints 9

Modern programming:

def print_error():
        print("Error: Operation string does not match operation object. Looks like you don't understand how our library works.")

class Operation:
    def get_a(self):
        print("This function must be overridden")
    def get_b(self):
        print("This function must be overridden")
    def __init__(self, typeStr):
        if self.type != typeStr:
            print_error()
        self.type = typeStr
    def perform_operation(self):
        if self.type == "+":
            return self.get_a() + self.get_b()

        if self.type == "-":
            return self.get_a() - self.get_b()

class Add(Operation):
    def get_a(self):
        return self.a
    def get_b(self):
        return self.b
    def __init__(self, a, b, typeStr):
        self.type = typeStr
        super().__init__(typeStr)
        self.a = a
        self.b = b

class Subtract(Operation):
    def get_a(self):
        return self.a
    def get_b(self):
        return self.b
    def __init__(self, a, b, typeStr):
        self.type = typeStr
        super().__init__(typeStr)
        self.type = "-"
        self.a = a
        self.b = b

def operation(a, b, operation, operationObject):
    if operation != operationObject.type:
        print_error()
        return
    if type(operationObject) == Add:
        value = operationObject.perform_operation()
        verification = Subtract(value, b, "-").perform_operation()
        if verification != a:
            print("There was an error verifying the result")
        return value

def add_two_numbers(a, b):
    operationObject = Add(a, b, "+")
    return operation(a, b, "+", operationObject)

print(add_two_numbers(4, 5)) #prints 9