Operator overloading : Instances of different classes

[Prasoon Shukla]

In Python, we have special methods that when used in a class definition, overload standard operators.

Now, here's the problem I am having (shown as an example):

Let us say we have a vector - which is an instance of a class that has been subclassed from Basic. Let's call this class Vector. Then, I have a container class, VectMul (subclassed from Mul), that is used to hold any objects of type scalar * vector - much like Mul does. I won't go too deep into what else VectMul class does.

Anyway, now, I have defined a method Vector.__mul__ which is supposed to return a VectMul after performing appropriate checks on the input.

At this point, let me just refresh your memory : if x and y are python objects, then:

x + y => x.__add__(y)

y + x => y.__add__(x)

Back to my question. So, when a user does:

• Vector*Vector, the Vector.__mul__ method will raise an error saying that a user cannot multiply two vectors.
  
• Vector*scalar, the Vector.__mul__ will be called and a VectMul will be returned.
• scalar*Vector : Now this is the problem - here let us say that scalar is a Add - then the method Add.__mul__ will be called instead of what I want to be called  - in this case, Vector.__mul__ - and return a Mul object instead of a VectMul.

This is the problem I face right now. I googled around a bit - apparently, we can use the __rmul__ method. But, for that to work - the Add.__mul__ (in the third point above) should return NotImplemented (what does that even mean? Raise an exception?). Obviously, I shouldn't have to change Add.__add__ for this. So, how can I handle this problem?

[Julien Rioux]

On Wednesday, 3 July 2013 10:47:04 UTC-4, Prasoon Shukla wrote:

Back to my question. So, when a user does:

• Vector*Vector, the Vector.__mul__ method will raise an error saying that a user cannot multiply two vectors.
  
• Vector*scalar, the Vector.__mul__ will be called and a VectMul will be returned.
• scalar*Vector : Now this is the problem - here let us say that scalar is a Add - then the method Add.__mul__ will be called instead of what I want to be called  - in this case, Vector.__mul__ - and return a Mul object instead of a VectMul.

This is the problem I face right now. I googled around a bit - apparently, we can use the __rmul__ method. But, for that to work - the Add.__mul__ (in the third point above) should return NotImplemented (what does that even mean? Raise an exception?). Obviously, I shouldn't have to change Add.__add__ for this. So, how can I handle this problem?

_op_priority and the @call_highest_priority are there exactly for this purpose. Define it in your class with a value above 10.0, which is Expr's default value for it. Also define __rmul__, using the decorator @call_highest_priority. This will allow your class to control what happens during multiplication (left or right).

Cheers,
Julien

[Frédéric Bastien]

Hi,

I think changing Add.__add__ to return NotImplemented is the right solution. This is the Python behavior for what you want. Python will raise an error if Add.__add__ return NotImplemented and y.__radd__(x) don't work.

There isn't any other way to do this to my knowledge.

Frédéric

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