This python program illustrates how to write a python program to convert infix expression to postfix using python list as stack

infix_to_postfix.py

def getOperatorWeight(op):
    weights = {
        '+': 1,
        '-': 1,
        '*': 2,
        '/': 2,
        '^': 3
    }
    return weights.get(op)
def isRightAssociative(op):
    return op == '^'
def isOperand(c):
    if c not in ['+','-','*','/','^','(',')','{','}']:
        return True
    return False
def isOperator(c):
    if c in ['+','-','*','/','^']:
        return True
    return False
def isEmpty(stack):
    return len(stack) == 0
def stackTop(stack):
    return stack[len(stack)-1]
def HasHigherPriority(op1,op2):
    op1_weight = getOperatorWeight(op1)
    op2_weight = getOperatorWeight(op2)
    if op1_weight == op2_weight:
        if isRightAssociative(op1_weight): return False
        return True
    return op1_weight > op2_weight

def ConvertToPostfix(exp):
    stack = []
    postfixString = ''
    for el in exp:
        if isOperand(el):
            postfixString += el
        elif el == "(":
            stack.append(el)
        elif isOperator(el):
            while not isEmpty(stack) and stackTop(stack) != '(' and HasHigherPriority( stackTop(stack), el):
                postfixString += stackTop(stack)
                stack.pop()
            stack.append(el)
        elif el == ")":
            while  not isEmpty(stack) and stackTop(stack) != '(':
                postfixString += stackTop(stack)
                stack.pop()
            stack.pop()
    while not isEmpty(stack):
        postfixString += stackTop(stack)
        stack.pop()
    return postfixString


def main():
    expression = "a+b*(c^d-e)^(f+g*h)-i"
    postfix = ConvertToPostfix(expression)
    print postfix # outout abcd^e-fgh*+^*+i-

if __name__ == '__main__':
    main()