create math expression parser

def parse(expr):
    stack = []
    for char in expr:
        if char not in ['+', '-', '*', '/']:
            stack.append(int(char))
        elif char == '-':
            left = stack.pop()
            right = stack.pop()
            stack.append(right - left)
        elif char == '+':
            left = stack.pop()
            right = stack.pop()
            stack.append(right + left)
        elif char == '*':
            left = stack.pop()
            right = stack.pop()
            stack.append(right * left)
        elif char == '/':
            left = stack.pop()
            right = stack.pop()
            stack.append(right / left)
    return stack.pop()

parse("231*+9-")