On Mon, Mar 3, 2014 at 11:52 PM, Steven D'Aprano <st...@pearwood.info> 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. I feel like 'is' is an operator that ought to be saved for an advanced course. Out of curiosity, do you think we should be doing truth checking with 'is'? True and False are singletons, and it seems to me that the justification for idenity versus equality should be just as strong there, but I don't think I've ever seen anyone even suggest that. -- Jerry -- https://mail.python.org/mailman/listinfo/python-list
