Code Fragment 6.4: Function for matching delimiters in an arithmetic expression.

match_delimiters.py

#!/usr/bin/env python


class Empty(Exception):
    pass


class ArrayStack:

    def __init__(self):
        self._data = []

    def __len__(self):
        return len(self._data)

    def is_empty(self):
        return len(self._data) == 0

    def push(self, e):
        self._data.append(e)

    def top(self):
        if self.is_empty():
            raise Empty('Stack is empty')
        return self._data[-1]

    def pop(self):
        if self.is_empty():
            raise Empty('Stack is empty')
        return self._data.pop()


def is_matched(expr):
    lefty = '({['
    righty = ')}]'
    S = ArrayStack()

    for c in expr:
        if c in lefty:
            S.push(c)
        elif c in righty:
            if S.is_empty():
                return False
            if righty.index(c) != lefty.index(S.pop()):
                return False
    return S.is_empty()


print(is_matched('{[()]}'))