[Python-Dev] redefining is

[Guido van Rossum]

> 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.

Sorry, if you're usign *any* immutable value there and expecting it to
be a unique object, you're cruisin' for a bruisin', so to speak.  The
language spec explicitly *allows* but does not *require* the
implementation to cache and reuse immutable values.

> 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. 

Please, just use None, [] or object() as a sentinel if you're going to
compare using 'is'.

--Guido van Rossum (home page: http://www.python.org/~guido/)