[Python-Dev] redefining is

[Jewett, Jim J]

Andrew Koenig:

> The observation is that if an object is immutable, there's 
> no legitimate reason to know whether it's distinct from
> another object with the same type and value.

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

"done with this data" might well appear in the input, but
the specific newly-created-string (which happens to look
just like that) can't appear.

The sentinal is usually a mutable object, but it is sometimes
a string indicating the object's meaning.  ("fail")  It is
surprising that some objects (like small integers) cannot be
used, but I don't think the answer is to make the entire
idiom unusable.

You could argue that they ought to be using (id(x) == id(y))
to emphasize that == isn't enough, but ... (x is y) seems
just as clear, and the reference manual (5.9) says that is 
tests for object identity. 

-jJ