andrew cooke wrote in news:33019705.1873.1333801405463.JavaMail.geo-discussion-forums@ynmm9 in gmane.comp.python.general:
> > hi, > > please, what am i doing wrong here? the docs say > http://docs.python.org/release/3.1.3/library/stdtypes.html#comparisons > "in general, __lt__() and __eq__() are sufficient, if you want the > conventional meanings of the comparison operators" but i am seeing > >> assert 2 < three > E TypeError: unorderable types: int() < IntVar() > > with this test: > > > class IntVar(object): > > def __lt__(self, other): > return self.value < other > > so what am i missing? > The part of the docs you are relying on uses the wording "in general", IOW, it is not saying that defining __eq__ and __lt__ will always be sufficient. In this case the expression "2 < three" is calling int.__lt__, which doesn't know how to comapre to an instance of your class so returns NotImplemented. At this point if you had defined a __gt__ method the interpreter would then try and call that having first switched the arguments around. But you didn't so a TypeError is raised. I'm afraid I couldn't find anywhere in the docs where that behaviour is described, I suspect I only know it from lurking on usenet for a number of years. The best description that I could find of the behaviour you are seeing is at: http://docs.python.org/py3k/reference/expressions.html#not-in There is a paragraph that contains: "... the == and != operators always consider objects of different types to be unequal, while the <, >, >= and <= operators raise a TypeError when comparing objects of different types that do not implement these operators for the given pair of types. ..." Perhapse the docs could be reworded to note that, to define a full set of comparisons between *different* types, you need to define a full set of special methods. Some links I found along the way: http://docs.python.org/release/3.1.3/library/constants.html? highlight=__lt__#NotImplemented http://code.activestate.com/recipes/576685/ http://docs.python.org/py3k/library/functools.html#functools.total_ordering Rob. -- http://mail.python.org/mailman/listinfo/python-list
