Trying to create a python program that changes infix notation (e.g: 2+3) into postfix notation (2 3 +) using a class in Python 3

I need to implement a function called postfix(exp) that takes a string exp containing a postfix math expression as input and returns the result as a float.

Here is what I have so far:

class Stack:

#Create a New Empty Stack

def __init__(self):

self.__S = []

def __str__(self):

return str(self.__S)

def push(self,x):

self.__S.append(x)

def pop(self):

return self.__S.pop()

def top(self):

return self.__S[-1]

def postfix(exp): #Needs to be implemented

if __name__ == "__main__":

print('Welcome')

print('Enter Exit to Quit')

input = input('Enter Expression')

while input != "exit":

result = Stack(input)

print("Result:", result)

if input == "exit":

break

here is an example output:

Welcome Enter Exit to Quit Enter Expression: 1 2 + Result: 3.0 Enter Expression: 2 3 + Result: 5.0 Enter Expression: exit