[Python-Dev] (not) redefining is

[Jewett, Jim J]

(me):
>> There is an idiom (I've seen it more in Lisp than in python) 
>> of creating a fresh object to act as a sentinel.

Greg Ewing:
> In those rare cases where None is also a legitimate value, it's not
> hard to manufacture some unique object and test for it with 'is'. You
> just have to remember that strings and integers are not necessarily
> unique, which I don't think is too hard to keep in mind.

That is the (occasionally surprising) state today.

Since there is no intention of ever actually mutating the object,
it is common (if mistaken) to assume that any new object -- even
an immutable -- should work.

If we changed "is", it would be even more difficult to create 
a guaranteed unique object.  Fortunately, Guido rejected the 
wholesale change.

I'm still a bit worried about a comment (which I can no longer 
find) suggesting a change for {}, so that

	>>> {} is {}  
      True

I don't see this in 2.3, and would be very surprised (and 
disappointed) if it started to happen in 2.4, precisely because 
dictionaries are mutable objects that should work.  Even worse, 

	>>> [] is []
	False

was the recommended idiom; the difference between [] and {} as 
unique objects is far from clear.  I personally like (and have 
used) Barry's suggestion of a new named object().  Unfortunately, 
object() is longer to type, not as backwards compatible, and 
places too emphasis on what the sentinel *is* rather than what 
is *isn't*.

-jJ