RPN simple

#!/usr/bin/env python3

import cmd, sys

class ReversePolishCalculator(cmd.Cmd):
    intro = "This is a simple RPN calculator."
    prompt = "Expr: "

    def onecmd (self, expr):
        if expr == "q" or expr == "quit":
            sys.exit(0)
        if expr == "?" or expr == "help":
            print("Enter a simple math expression in Reverse Polish Notation, such as n1 n2 op, where n(1,2) are numbers and op is one of + - * /.")
            return

        stack = []
        tokens = expr.split(" ")
        for token in tokens:
            try:
                # Append to stack as a number -- value
                stack.append(float(token))
            except Exception as e:
                # Can't convert to float -- operator?
                if len(stack) < 2:
                    print("Syntax Error. Try n1 n2 op, where n1 and n2 are numbers and op is one of + - * /.")
                    stack = []
                    return
                if token == "+":
                    stack.append(stack.pop() + stack.pop())
                elif token == "-":
                    stack.append(stack.pop() - stack.pop())
                elif token == "*":
                    stack.append(stack.pop() * stack.pop())
                elif token == "/":
                    divisor, dividend = stack.pop(), stack.pop()
                    stack.append(dividend / divisor)
                else:
                    print("Only operations +-*/ are supported.")
                    stack = []
                    return

                print("==> {}".format(stack.pop()))
                if len(stack) > 0:
                    stack = []
                    print("[i] Only two operands are evaluated.")

if __name__ == '__main__':
    ReversePolishCalculator().cmdloop()