python 逆波兰表达式练习

"""
逆波兰表达式练习
"""


class StackError(Exception):
    pass


# 顺序栈类
class SStack:
    def __init__(self):
        # 空列表 就是栈的存储空间
        # 列表的最后一个元素作为栈顶
        self._elems = []

    # 是否为空
    def is_empty(self):
        return self._elems == []

    # 入栈
    def push(self, elem):
        self._elems.append(elem)

    # 出栈
    def pop(self):
        if self.is_empty():
            raise StackError('Stack is empty')
        return self._elems.pop()

    # 查看栈顶元素
    def top(self):
        if self.is_empty():
            raise StackError('Stack is empty')
        return self._elems[-1]


# 初始化栈
st = SStack()
while True:
    a = input("")
    s = a.split(' ')
    for item in s:
        if item in ['+', '-', '*', '/']:
            a1 = st.pop()
            a2 = st.pop()
            st.push(str(eval（a2 + item + a1)))
        elif item == 'p':
            print(st.top())
        else:
            st.push(item)