to_postfix.py

tokens = ['111', '+', '2', '+', '(', '33', '**', '2', ')', '-', '1', '*', '1']
# 111 2 + 33 2 ^ + 1 1 * -

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

def to_postfix(tokens):
    operators = {
        '**': 3,
        '*': 2,
        '/': 2,
        '+': 1,
        '-': 1
    }
    
    output_queue = []
    operator_stack = []
    
    for t in tokens:
        if is_number(t):
            output_queue.append(t)
        elif t in operators:
            if operator_stack:
                on_top = operator_stack[-1]
                if on_top == '(':
                    operator_stack.append(t)
                elif operators[on_top] >= operators[t]:
                    output_queue.append(operator_stack.pop())
                    operator_stack.append(t)
                else:
                    operator_stack.append(t)
            else:
                operator_stack.append(t)
        elif t == '(':
            operator_stack.append(t)
        elif t == ')':
            while operator_stack:
                on_top = operator_stack[-1]
                if on_top == '(':
                    operator_stack.pop()
                    break
                else:
                    output_queue.append(operator_stack.pop())
    
    while operator_stack:
        output_queue.append(operator_stack.pop())

    return output_queue

print(to_postfix(tokens))