How to redirect operation methods to some sepcific method easily?

[Victor Lin]

Hi,
I'd like to write some class that can help me build reusable formula
easily, some simple code like this.

# -*- coding: utf8 -*-

class OperationResult:
    def __init__(self, left, right):
        self.dataSource = dataSource

    def __add__(self, other):
        self.dataSource.stack.append('+')
        return self

    def __div__(self, other):
        self.dataSource.stack.append('/')
        return self

class Operand:
    def __init__(self, dataSource, name):
        self.dataSource = dataSource
        self.name = name

    def __add__(self, other):
        self.dataSource.stack.append(self)
        self.dataSource.stack.append(other)
        self.dataSource.stack.append('+')
        return OperationResult(self.dataSource)

class DataSource:
    def __init__(self):
        self.stack = []

    def __getitem__(self, key):
        return Operand(self, key)

def ROE(dataSource):
    a = dataSource[u'股東權益總額']
    b = dataSource[u'每股淨額']
    c = dataSource[u'當季平均市價']
    return (a + b) / c

Now I can use ROE formula easily, it will return a object that contain
formula stack like this
[a, b, +, c, /]

And then I can get real data of those items from database or other
place. The formula depends on nothing. It don't care where to get data
and how to calculate it. I can reuse it easily.

Now, here comes the problem : I have to override all the operation
methods, such as __add__ and __mul__.
I know the most stupid way is just to write all of them like this.

def __add__(self, other):
     self.leftOperation('add', other)

def __mul__(self, other):
     self.leftOperation('mul', other)

But I don't want to do that in this stupid way. I want to use little
code to redirect these operation methods to some specific method with
it's name.

What can I do?

Thanks.