Python operator overloading : commutative or right only?

Question

How is the operator overloading parsed? I have a class object o and want to do -1 * o with the overloaded __mul__ operator. Would that be parsed correctly when the left operand is a -1? Multiplication should be commutative (except for matrices and cross-products)... ?

FYI: Multiplication is overloaded by defining __mul__, not __mult__. — jwodder
– jwodder, Commented Oct 6, 2014 at 18:05
Did you read the documentation, which describes separate methods for customizing left- and right-hand operations? — BrenBarn
– BrenBarn, Commented Oct 6, 2014 at 18:06
Not all multiplication is commutative. Matrix multiplication, for instance. That's why there's separate left and right multiplication operators. — khelwood
– khelwood, Commented Oct 6, 2014 at 18:11
Yes matrix or vector vector mult is not. I was looking at scalar - vector products in my case. — El Dude
– El Dude, Commented Oct 6, 2014 at 18:17

kindall · Accepted Answer · 2014-10-06 18:06:39Z

That's what __rmul__ is for. In your scenario, Python calls int.__mul__(-1, o). int doesn't know how to do this operation, so this call returns NotImplemented. Python therefore calls type(o).__rmul__(o, -1) giving your class the chance to handle it.

jwodder · Accepted Answer · 2014-10-06 18:05:02Z

-1

Operators are always parsed the same way regardless of overloading. Because unary - has higher precedence than *, -1 * o will always be parsed as (-1) * o.

answered Oct 6, 2014 at 18:05

jwodder

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1 Comment

BrenBarn Over a year ago

I don't think that is what the OP is getting at, although his use of "parsing" may misleadingly suggest it.

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