rdparser.py

OPS = {
    '+': int.__add__,
    '-': int.__sub__,
    '*': int.__mul__,
    '/': int.__div__,
}

def evaluate(expression):
    def add(chars):
        e = mult(chars)        
        while chars[0] in "+-":
            e = OPS[chars.pop(0)](e, mult(chars))
        return e
 
    def mult(chars):
        e = primary(chars)        
        while chars[0] in "*/":
            e = OPS[chars.pop(0)](e, mult(chars))
        return e

    def primary(chars):
        if chars[0].isdigit():
            result = 0
            while chars[0].isdigit():
                result *= 10
                result += int(chars.pop(0))
            return result
        if chars[0] == '(':
            chars.pop(0) #(
            e = add(chars)
            chars.pop(0) #)
            return e
 
    return add(list(expression.replace(' ', '')+'$'))
    
print evaluate('2 * ( 2 + 1 )')