Puzzled by "is"

[Dick Moores]

At 08:23 AM 8/12/2007, Steve Holden wrote:
>Dick Moores wrote:
> > So would a programmer EVER use "is" in a script?
>
>Sure. For example, the canonical test for None uses
>
>      x is None
>
>because there is only ever one instance of type Nonetype, so it's the
>fastest test. Generally speaking you use "is" to test for identity (do
>these two expressions reference the same object) rather than equality
>(do these two expressions evaluate to equivalent objects).

Off the top of your head, could you or others give me as many 
examples as you can think of?

Thanks again,

Dick