frozenset() without arguments should return a singleton

[Terry Reedy]

"Stefan Behnel" <stefan.behnel-n05pAM at web.de> wrote in message 
news:cuj06r$ep5$1 at lnx107.hrz.tu-darmstadt.de...
> Hi!
>
> frozenset() doesn't behave as the other immutable empty data types in 
> 2.4:
>
> .>>> '' is ''
> True
> .>>> () is ()
> True

I believe the reference manual describes this sort of behavior for 
immutables as an *optional* optimization.  0 is 0 and has been since at 
least 1.3.  I am not sure both of your statements above have been true as 
long, but I not longer have 1.3 to check ;-).

> .>>> frozenset() is frozenset()
> False

> frozenset() called without arguments (or on empty sequences)
> should always return a singleton object.

If we interpret 'should' as 'preferably by me', ok.

>It is immutable, so I can't see a reason why it should take up more 
>resources than necessary.

It will take some programmer's time to add the special case check and run 
the test suite, and check in the changes.  Yours?  And perhaps some 
execution time for each frozenset call.  Since frozenset is not much used, 
and multiple empty frozensets very rare, and the difference mostly 
invisible, the frozenset implementor probably went on to other things.

Terry J. Reedy