Something weird about re.finditer()

[Steven D'Aprano]

On Sat, 18 Apr 2009 12:37:09 +1200, Lawrence D'Oliveiro wrote:

> In message <pan.2009.04.15.09.14.56 at REMOVE.THIS.cybersource.com.au>,
> Steven D'Aprano wrote:
> 
>> BTW, testing for None with == is not recommended, because one day
>> somebody might pass your function some strange object that compares
>> equal to None.
> 
> Presumably if it compares equal to None, that is by design, precisely so
> it would work in this way.

In context, no. We're not talking about somebody creating an object which 
is equivalent to None when treated as a value, but using None as a 
sentinel. Sentinels are markers, and it is important that nothing else 
can be mistaken for that marker or breakage will occur.

Of course, if the caller knows how the sentinel is used, then he might 
choose to duplicate that usage but pass some other object. But that would 
be stupid and should be discouraged. I mean, what would be the point? I 
can think of use-cases for creating something that returns equal to None 
-- the Null object pattern comes to mind. But what would be the point of 
creating an object that was not None but would fool a function into 
treating it as the same sentinel as None?



-- 
Steven