Question

I am trying to implement a simple expression evaluator in python,but I am stuck in parser method.Below is my code snippet.

    class Number:
    def __init__(self,value):
        self.value=value
    def execute(self):
        return self.value

class Plus:
    def __init__(self,left,right):
        self.left=left
        self.right=right
    def execute(self):
        return self.left+self.right

class Minus:
    def __init__(self,left,right):
        self.left=left
        self.right=right
    def execute(self):
        return self.left-self.right

class Multiply:
    def __init__(self,left,right):
        self.left=left
        self.right=right
    def execute(self):
        return self.left*self.right
import re
def parser(input):
    stack=[]
    token_pat = re.compile("\s*(?:(\d+)|(.))")
    for number, operator in token_pat.findall(input):


        if number:
            stack.append(Number(int(number)))

        else:
            first,second=stack.pop(),stack.pop()

            if operator=="+":
                stack.append(Plus(first,second))
            elif operator=="-":
                stack.append(Minus(first,second))
            elif operator=="*":
                stack.append(Multiply(first,second))
            else:
                raise SyntaxError("unknown operator")
    print stack[0].execute()


if __name__=="__main__":
    parser('1 2 +')

When I am running the above code,I am getting following error.Can anybody review my code .

Traceback (most recent call last):
  File "Interpreter.py", line 52, in <module>
    parser('1 2 +')
  File "Interpreter.py", line 48, in parser
    print stack[0].execute()
  File "Interpreter.py", line 12, in execute
    return self.left+self.right
TypeError: unsupported operand type(s) for +: 'instance' and 'in

Answer 1

The error message is that confusing because you’re using classic classes. With new-style classes (i.e. inheriting from object), you get a much more reasonable:

TypeError: unsupported operand type(s) for +: 'Number' and 'Number'

Either use ints instead of Number objects, or implement adding logic by implementing __add__ and friends, or add evaluating logic in Plus.execute.

Also note that you’re reimplementing Python’s built-ins. Additionally, having an execute method is pretty much an anti-pattern. A much shorter implementation would be

import functools,operator,re

def parser(inp):
    stack=[]
    token_pat = re.compile("\s*(?:(\d+)|(.))")
    for number, op in token_pat.findall(inp):
        if number:
            stack.append(functools.partial(lambda i:i, int(number)))
        else:
            first,second=stack.pop(),stack.pop()
            try:
                op = {
                    '+': operator.add,
                    '-': operator.sub,
                    '*': operator.mul
                }[op]
            except KeyError:
                raise SyntaxError("unknown operator")

            stack.append(functools.partial(lambda op,first,second:
                        op(first(), second()), op, first, second))
    print(stack[0]())

if __name__=="__main__":
    parser('1 2 + 3 *')