expr_parse.py

import re

def expr_parse(exp):
    assert isinstance(exp, str)
    XRE = re.compile(r'(?:(?<=\D)(?=\d)|(?!=\d))', re.MULTILINE)
    return re.sub(XRE, " ", exp)


class EmptyStackException(Exception):
    pass

class Stack:
    def __init__(self):
        self._list = []

    def push(self, item):
        self._list.append(item)

    @property
    def isEmpty(self):
        return self._list == []

    def pop(self):
        if self.isEmpty:
            raise EmptyStackException("Stack is empty")
        else:
            return self._list.pop()

    def peek(self):
        if self.isEmpty:
            return None
        else:
            return self._list[-1]

def convertToPostFix(expStr):
    tokens = expr_parse(expStr).split(' ')
    OPS = ("+", "-", "*", "/", "(", ")")
    precs = {
        "*" : 2,
        "/" : 2,
        "+" : 1,
        "-" : 1,
        "(" : 0,
    }
    opStack = Stack()
    output = []

    for item in tokens:
        if item not in OPS:
            output.append(item)
        elif item == "(":
            opStack.push(item)
        elif item == ")":
            while opStack.peek() != "(":
                output.append(opStack.pop())
            opStack.pop()
        else:
            done = False
            while not opStack.isEmpty and not done:
                if precs[opStack.peek()] >= precs[item]:
                    output.append(opStack.pop())
                else:
                    done = True
            opStack.push(item)
    while not opStack.isEmpty:
        output.append(opStack.pop())

    return " ".join(output)

#a = "12+3*8"
a = "(26+46)*(89-12)"
print(convertToPostFix(a))