question2.py

#NB used python3
#2 Create a simple calculator
def eval_helper(expression, index=0):
    result=0
    op='+'
    while index < len(expression):
        char=expression[index]
        if char.isdigit():
            if op == '+':
                result += int(char)
            else:
                result -= int(char)
        elif char in('+','-'):
            op = char
            
        elif char == '(':
            n, index = eval_helper(expression, index + 1)
            if op == '+':
                result += n
            else:
                result -= n
        elif char == ')':
            return(result, index)
            
        index += 1
    return(result, index)

def eval(expression):
    return eval_helper(expression)[0]
    
#print (eval('-(3+(2-1))'))