Operator commutativity

Mark Dickinson dickinsm at gmail.com
Wed Sep 21 19:03:15 CEST 2011


On Sep 21, 2:07 am, Steven D'Aprano <steve
+comp.lang.pyt... at pearwood.info> wrote:
> After playing around with various combinations of C1, C2, D1 and D2, it
> seems to me that the rule is:
>
> If the right-hand argument is a subclass of the left-hand argument, AND also
> defines __radd__ directly rather than inheriting it, then its __radd__
> method is called before the left-hand argument's __add__ method.
>
> which strikes me as a strangely specific and not very useful rule. I suspect
> it might be an accident of implementation rather than a deliberate feature.

I'm 99.9% sure it's deliberate rather than an accident of
implementation.  See the note in the docs at:

http://docs.python.org/reference/datamodel.html#emulating-numeric-types

Support that you're subclassing int, (class MyInt(int): ...) and you
want to intercept additions of the form 3 + MyInt(4)  (perhaps because
you want MyInt to be 'contagious', so that an arithmetic operation
that combines an int and a MyInt returns a MyInt).  How would you
achieve this without this rule?

--
Mark



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