Why does my postfix program in Python not work?

Question

I have this code to calculate something given in postfix notation:

operator = ["+", "-", "*", "/", "^"]
operand=[]
exp=input("Please enter something in postfix notation:")
expression = exp.split(' ')
for ele in expression:
        if(ele not in operator):
            operand.append(ele)
        else:
            number1=eval(operand.pop())
            number2=eval(operand.pop())
        if(ele=="+"):
            answer=number2+number1
        elif(ele=="-"):
            answer=number2-number1
        elif(ele=="*"):
            answer=number2*number1
        elif(ele=="/"):
            answer=number2/number1
        elif(ele=="^"):
            answer=number2**number1
        operand.append(str(answer))
print("answer =", operand[0])

I can enter something to calculate but then it says NameError: name 'answer' is not defined and refers to the operand.append(str(answer)) line. How do I fix that and is the rest fine? Thank you for any help!

Answer 1

Your indentation isn't correct in your else block, which is appending garbage value for the answer at every element.

operator = ["+", "-", "*", "/", "^"]
operand=[]
exp=input("Please enter something in postfix notation:")
expression = exp.split(' ')

for ele in expression:
        if(ele not in operator):
            operand.append(ele)
        else:
            number1=eval(operand.pop())
            number2=eval(operand.pop())
            if(ele=="+"):
                answer=number2+number1
            elif(ele=="-"):
                answer=number2-number1
            elif(ele=="*"):
                answer=number2*number1
            elif(ele=="/"):
                answer=number2/number1
            elif(ele=="^"):
                answer=number2**number1
            operand.append(str(answer))

print("answer =", operand[0])

Here's your updated code.

Following is the output:

Please enter something in postfix notation:15 5 +
answer = 20