why () is () and [] is [] work in other way?

[Terry Reedy]

On 4/21/2012 9:02 AM, Bernd Nawothnig wrote:

> You should better not rely on that result. I would consider it to be
> an implementation detail. I may be wrong, but would an implementation
> that results in
>
> () is () ==>  False
>
> be correct or is the result True really demanded by the language
> specification?

To make sure that the result is not due to ref counting garbage 
collection in the middle of the expression, one must test like so:
 >>> a=()
 >>> b=()
 >>> a is b
True

This is explicitly an implementation detail, something that *may* be 
done. I am not sure if the above has always been the case for CPython. 
CPython's set of pre-built ints has been expended. I think string 
interning has also changed.

-- 
Terry Jan Reedy