[Python-Dev] redefining is

[Phillip J. Eby]

At 05:40 PM 3/19/04 -0500, Andrew Koenig wrote:

>Nevertheless, I still wish that expressions such as "x is 'foo'" did not
>silently differ in outcome from one implementation to another.

The part that drives me nuts about this discussion is that in my view, "x 
is 'foo'" has the *same* outcome on all implementations.  That is, it's 
true if x refers to that exact string object.

The thing that's different from one implementation to the next is whether 
there's any chance in hell of x being that same 'foo' string.  But to me, 
that 'foo' string looks like a *newly created* string, so to the naive 
glance there's no possible way that it could be the same object.  In other 
words, it looks like a bad expression to use in the first place: one that's 
guaranteed to be false, except by accident of implementation.

So, I have trouble understanding how it is that somebody could get to a 
place where they think that using 'is' for strings and numbers is a good 
idea in the first place.  But then, I read the entire Python language 
reference (and a good chunk of the library reference) before I tried 
writing even a single line of Python code, so I can imagine that my 
perspective on this might not be the most common one.  :)