On Thursday, October 23, 2014 11:46:02 AM UTC-7, João Alberto Ferreira wrote: > > I am running the following Python example from the book "Learning > Python", from Mark Lutz and David Ascher, but Sage is returning a > TypeError after presenting the correct response. Can anyone explain me > why? I've found this very strange. > > sage: class Commuter: > ....: def __init__(self, val): > ....: self.val = val > ....: def __add__(self, other): > ....: print "add", self.val, other > ....: def __radd__(self, other): > ....: print "radd", self.val, other > ....: > sage: x = Commuter(88) > sage: y = Commuter(99) > sage: x + 1 > add 88 1 > sage: 1 + y > radd 99 1 >
The problem is caused by the fact that your __radd__ implementation returns "None". If you insert a return command for a non-None value, the example works as expected. Indeed Sage binary operations internally do not dispatch via __add__ and __radd__, but __radd__ is tried as a fall-back at some point. If you call 1+y then you're running Integer(1).__add__(y). This does not figure out a way to do the addition. Apparently, sage chooses to not return NotImplemented (allowing python to call __radd__), but calls __radd__ itself and still raises an error when the return value is None. There may be a good reason why Sage chooses to call __radd__ manually rather than let python do the work by returning NotImplemented, but if there's not perhaps it would be better to stay closer to python's standard? -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To post to this group, send email to sage-support@googlegroups.com. Visit this group at http://groups.google.com/group/sage-support. For more options, visit https://groups.google.com/d/optout.
