Evaluating arithmetic expression with recursion.

[muddybucket]

Hi, I am tasked to make a recursive function which evaluates the expression starting from the given position. Return the value and the first position after the read sub-expression. If the string starting at the given expression is not an arithmetic expression, return None. To avoid divisions by zero we do not allow division in arithmetic expressions.

• If a is a number, then evaluate(a) is that number.
  
• If a and b are arithmetic expressions, then evaluate((a+b)) = evaluate(a)+evaluate(b), evaluate((a-b)) = evaluate(a)-evaluate(b), and evaluate((a*b)) = evaluate(a)*evaluate(b).
  

def read_positive_integer(text, position):
    text_mod = text[position:]
    if not (text[position]).isdigit():
        return None
    try:
        return (int(text_mod), len(text_mod) + position )
    except:
        return read_positive_integer(text[:-1], position)

def evaluate(expression, position):
    r = read_positive_integer(expression, position)
    if r != None:
        return r

    else:
        if expression[position] == "(":
            a,pos = evaluate(expression, position+1)
            operator = expression[pos]
            b,pos = evaluate(expression, pos+1)
            op_dict = {"+":a+b, "-":a-b, "*":a*b}

            if expression[pos] == ")":
                return op_dict[operator] 

I have tried giving it the following input:
evaluate("((1-1)+(2*1))", 0)
But it outputs this error:

Error:
cannot unpack non-iterable int object

PD: The function read_positive_integer has no flaws and it works as it should (it got accepted by the auto-corrector)