On Tuesday, March 4, 2014 8:49:22 PM UTC+5:30, Jerry Hill wrote: > On Mon, Mar 3, 2014 at 11:52 PM, Steven D'Aprano wrote: > > If your intention is to treat None as a singleton sentinel, not as a > > value, then you ought to use "is" to signal that intention, rather than > > using == even if you know that there won't be any false positives.
> In all of the years I've been on this list, I don't think I've seen > more than one or two cases of someone deliberately treating None as a > singleton sentinel. In most cases, they're either checking the return > value from a function or using it as a default argument to a function > to force some default behavior when no parameter is passed. I'm > pretty sure you're going to say that the latter use is exactly where > you should us 'is' instead of '=='. Respectfully, I disagree. > For a beginning python user, identity checking is an attractive > nuisance. The only time most beginners should use it is when comparing > to None. But, as soon as they are taught that there are two > comparison operators, I start to see 'is' cropping up in more and more > places where they ought to use '=='. And the problem is that > sometimes it works for them, and sometimes it doesn't. Sure, students > eventually need to understand the difference between identity and > equality. My problem is that by enshrining in python custom that the > only correct way to compare to None is with 'is', we have to explain > that concept way early in the teaching process. I can't count the > number of times that a thread has completely derailed into identity vs > equality, then into interning of strings and small integers, and > suddenly the thread is 40 messages long, and no one has actually > talked about the code that was originally posted beyond that issue. > In approximately zero cases, have I seen code where 'is' versus '==' > actually made any difference, except where the 'is' is wrong. I've > also never seen the supposedly ever-present boogie man of an object > that mistakenly compares equal to None, much less seen that object > passed to functions with None-based sentinels. Beautifully put -- thanks Jerry! > I feel like 'is' is an operator that ought to be saved for an advanced course. +100 My choice: have an isNone function which should take care of 90% of the cases of valid 'is' (if I got Chris' statistics right) and avoid most of the metaphysical BS about identity -- https://mail.python.org/mailman/listinfo/python-list
