On Sep 21, 2:07 am, Steven D'Aprano <steve +comp.lang.pyt...@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 -- http://mail.python.org/mailman/listinfo/python-list
