Python实现括号匹配检查的代码中横线处应填入的语句是？

采用如下代码实现检查输入的字符串括号是否匹配，横线上应填入的代码为（）。

class Stack:
    def __init__(self):
        self.items = []
    def is_empty(self):
        return not self.items
    def push(self, item):
        self.items.append(item)
    def pop(self):
        if not self.is_empty():
            return self.items.pop()
    def peek(self):
        if not self.is_empty():
            return self.items[-1]
    def size(self):
        return len(self.items)

def paren_match(expr):
    s = Stack()
    balanced = True
    index = 0
    while index < len(expr) and balanced:
        symbol = expr[index]
        if symbol in '([{':
            ________________
        else:
            if s.is_empty():
                balanced = False
            else:
                top = s.pop()
            if not matches(top, symbol):
                balanced = False
        index += 1

    if balanced and s.is_empty():
        return True
    else:
        return False

def matches(opening, closing):
    opens = '([{'
    closers = ')]}'
    return opening in opens and closers.index(closing) == opens.index(opening)

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