STSPBWBBIWHTD

#scan the infix string from left to right
#initialise an empty stack
#if the scanned character is an operand then add it to the postix string
#if the scanned character is an operator and the stack is empty, push the character to the stack
#if the scanned character is operator and stack not empty, compare the precedence of the character with the elemont on top of the stack, if the top of stack has higher precedence over the scanned character then pop the stack else push the scanned character to stock, repeat this step as long as stack is not empty and top stack has precedence over the character.
#repeat until all characters are scanned.
#pop contents of the stack 1 by 1 onto the end of the postfix string until the stack is empty
#return the postfix string
def inToPost(infix):
    #initialise empty stack and output
    postfix = ""
    stack = []

    #scan each character left to right
    for count in range(0, len(infix)):
        token = infix[count]
        #print token
        #print postfix
        #print stack
        if isNumber(token):
            #if scanned character is an operand then add to postfix string
            postfix += token
        elif isOperator(token):
            #scanned character is an operator

            comparing = True
            while comparing == True:
                if len(stack) == 0:
                    #if stack is empty, push token onto the stack
                    stack.append(token)
                    comparing = False
                elif precedence(token) >= precedence(stack[-1]):
                    stack.append(token)
                    comparing = False
                else:
                    postfix += stack.pop()


            '''
            else:
                #compare precedence
                comparing = True
                while comparing == True:
                    if precedence(stack[-1]) <= precedence(token):
                        #top of stack has lower precedence, add token to output
                        comparing = False
                        postfix += token
                    else:
                        #top of stack has higher precedence, add top of stack to output
                        postfix += stack.pop()
                        if len(stack) == 0:
                            stack.append(token)
                            comparing = False
                    print postfix
                #end of while
            '''
            #end of isOperator
        else:
            print "invalid token: " + token + "discarded"
        #print "\n"
        #print postfix
        #print stack
        #print "\n###~###\n"
    #pop contents of stack onto postfix string
    for count in range(0, len(stack)):
        postfix += stack.pop()
    return postfix

def isNumber(token):
    try:
        token = int(token)
        return True
    except:
        return False

def isOperator(token):
    if token in ["+", "-", "*", "/", "^"]:
        return True
    else:
        return False

def precedence(token):
    #bidmas
    if token == "-":
        return 1
    elif token == "+":
        return 1
    elif token == "*":
        return 2
    elif token == "/":
        return 2
    elif token == "^":
        return 3
    return 0

expression = "1+2*3-4"
#expression = "3+4*5/6"
print inToPost(expression)