eval_postfix.py

postfix = ['111', '2', '+', '33', '2', '**', '+', '1', '1', '*', '-']

def is_number(num):
    # quick way
    return num[0] in '0123456789'

def eval_postfix(postfix):
    stack = []
    for element in postfix:
        if is_number(element):
            stack.append(element)
        else:
            v1 = int(stack.pop())
            v2 = int(stack.pop())
            if element == '**':
                v = v2 ** v1
                stack.append(v)
            elif element == '*':
                v = v2 * v1
                stack.append(v)
            if element == '/':
                v = v2 // v1
                stack.append(v)
            if element == '+':
                v = v2 + v1
                stack.append(v)
            if element == '-':
                v = v2 - v1
                stack.append(v)
                
    return stack[0]
            
print(eval_postfix(postfix))