On 29 Jan, 06:56, Terry Reedy <tjre...@udel.edu> wrote: > On 1/28/2010 6:47 PM, Paul Boddie wrote: > > > What would annoy me if I used Python 3.x would be the apparent lack of > > the __cmp__ method for conveniently defining comparisons between > > instances of my own classes. Having to define all the rich comparison > > methods frequently isn't even as much fun as it sounds. > > You define __eq__, which automatically generates __ne__.
>From the Python 3.1 language reference: "There are no implied relationships among the comparison operators. The truth of x==y does not imply that x!=y is false. Accordingly, when defining __eq__(), one should also define __ne__() so that the operators will behave as expected." Maybe Python 3.1 plugs a default method in, anyway. > You define __lt__, which is all sort and heap need. > This should be about as easier than __eq__, which is really needed, and > __cmp__. If you need the other 3, either copy the recipe in the > Cookbook, or, even faster > > def __ge__(s,o): return o.__lt__(s) > def __le__(s,o): return s < o or s == o > def __ge__(s,o): return s > o or s == o Spot the error in the above. ;-) Of course, this could be defined in a base class and be inherited everywhere, but the case that made me raise this issue actually involved instances of a class delegating comparison/sorting to another object. With __cmp__ this can be done concisely and only involve the delegation of one method - not so with the rich comparison "protocol". Paul -- http://mail.python.org/mailman/listinfo/python-list
