gistfile1.py

def crazy_computation(expression, values):
    """
    given expression = ["a", "+", "b", "-", "c"] and values = {"a": [1, 2, 3], "b": [4, 5, 6], "c": [7, 8, 9]},
    return [-2, -1, 0]
    """
    
    # taking care of the cases when one of the two first operands is a list
    
    if not isinstance(expression[0], basestring):
        intermediate = crazy_computation(expression[0], values)
        return crazy_computation([intermediate] + expression[1:], values)
    
    if not isinstance(expression[2], basestring):
        intermediate = crazy_computation(expression[2], values)
        return crazy_computation(expression[:2] + intermediate + expression[3:], values)
        
    # now the first two operands are actual values.
    
    intermediate = operate(expression[0], expression[1], expression[2])
    
    if len(expression) == 3:
        return intermediate
    else:
        return crazy_computation([intermediate], expression[3:])
        
def operate(a, op, b):
    op_dict = {
        "+": operator.add,
        "-": operator.sub,
        "*": operator.mul,
        "/": operator.div,
    }
    return [op_dict[op](x, y) for x, y in zip(a, b)]