Steven D'Aprano wrote: > On Fri, 23 Jan 2009 20:33:45 -0500, Steve Holden wrote: > >> Steven D'Aprano wrote: >>> On Fri, 23 Jan 2009 14:58:34 -0500, Gerald Britton wrote: >>> >>>> Hi -- Some time ago I ran across a comment recommending using <var> is >>>> None instead of <var> == None (also <var> is not None, etc.) >>> That entirely depends on whether you wish to test for something which >>> *is* None or something with *equals* None. Those two things have >>> different meanings. >>> >> No they don't, because the language *guarantees* the None object is a >> singleton, so anything that *equals* None *is* None. > > Twice in one day. Have they put funny chemicals in the water over there? > *wink* > Nope, but you know what newsgroup response propagation is like ...
> Steve, in case you missed my earlier response: > >>>> class Empty: > ... def __eq__(self, other): > ... return not bool(other) > ... >>>> e = Empty() >>>> e == None > True >>>> e is None > False > > > An instance that compares equal to anything false doesn't strike me as > particularly bizarre or pathological. For instance, the Python Cookbook > has an implementation for the Null object pattern. The implementation > given compares unequal to everything, but suggests defining an > appropriate __eq__ if you need different behaviour. > Sure, my syllogism was not strictly true, hence my final quote: > Of course there can be pathological objects with bizarre comparison > methods. And the "is" test helps avoid them. Personally I believe that the Empty class is at least slightly pathological. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
