python - 分流場アルゴリズムの複雑な式で電卓エラーが発生する

Question

ここに見られるように、私は分流場アルゴリズムを実装しました:

#!/usr/bin/env python

import sys
import string
import operator
import signal

class InvalidStackError(Exception): pass
class BadParenError(Exception): pass

def hook_ctrl_c(signal, frame):
    print ""
    sys.exit(0)

signal.signal(signal.SIGINT, hook_ctrl_c)

ops = {
    "*" : [operator.mul, 3, "left"],
    "x" : [operator.mul, 3, "left"],
    "/" : [operator.div, 3, "left"],
    "+" : [operator.add, 2, "left"],
    "-" : [operator.sub, 2, "left"],
    "^" : [operator.pow, 4, "right"],
    "(" : [None, 5, "neither"],
    ")" : [None, 5, "neither"]
}

def parse(raw):
    return raw.split()

def compile_to_rpn(tokens):
    operator_stack = []
    rpn_code = []

    for token in tokens:
        try:
            current_number = int(token)
            rpn_code.append(current_number)

        except ValueError:
            if token == "(":
                operator_stack.append(token)

            elif token == ")":
                try:
                    while True:
                        current_operator = operator_stack.pop()

                        if current_operator == "(":
                            break

                        rpn_code.append(current_operator)

                except IndexError:
                    print "(error) mismatched parens"

            elif token in ops:
                if len(operator_stack) > 0 and ((ops[token][2] == "left" and ops[token][1] <= ops[operator_stack[-1]][1]) or (ops[token][2] == "right" and ops[token][1] < ops[operator_stack[-1]][1])):
                    operator = operator_stack.pop()

                    if operator != "(":
                        rpn_code.append(operator_stack.pop())

                    else:
                        operator_stack.append("(")

                operator_stack.append(token)
    try:
        while len(operator_stack) != 0:
            current_operator = operator_stack.pop()

            if current_operator in ["(", ")"]:
                raise BadParenError

            rpn_code.append(current_operator)

    except BadParenError:
        print "(error) mismatched parens"

    return rpn_code

current_token = None

while True:
    try:
        tokens = parse(raw_input("-> "))
        stack = []

        if tokens == ["quit"]:
            sys.exit(0)

        tokens = compile_to_rpn(tokens)

        for token in tokens:
            current_token = token

            if token not in ops:
                stack.append(int(token))

            else:
                rhs, lhs = stack.pop(), stack.pop()
                stack.append(ops[token][0](lhs, rhs))

        if len(stack) > 1:
            raise InvalidStackError

        print "Result: %s\n" % stack[0]

    except ValueError:
        print "(error) token {%s} is a not a number or operator\n" % current_token

    except IndexError:
        print "(error) expression has insufficient number of values\n"

    except InvalidStackError:
        print "(error) too many values\n"

「3 + 4」などの単純な式には問題なく機能しますが、複雑なものを入力すると、次のようになります。

$ ./rpn.py

-> 4 - 5 * 6 + 3 ^ 2

(エラー) 値が多すぎます

事前に感謝します。助けていただければ幸いです。