[Python-3000] sets in P3K?

[Raymond Hettinger]

Brian Harring wrote:

>Potentially a stupid question, but I've been following this thread for 
>a while and I'm still not seeing the real gain of extending the syntax 
>for frozen markers/set literals.
>
>Someone care to recap what actually is gained from having a set 
>literal?
>~harring
>  
>

Good question.  To see the answer, look at a code tranformation from:

    if file_ext.lower() in set(['html', 'xml', 'xhtml']):
        handle(filename)

into:


    if file_ext.lower() in {'html', 'xml', 'xhtml'}:
        handle(filename)

The latter form builds a set directly and does not construct a temporary 
intermediate list.  Also, the latter form is potentially optimizable so 
that the set is built at compile-time and referrenced as a constant 
rather than being rebuilt from scratch everytime the code is executed.

In terms of user-friendliness, the latter form is closer to the way sets 
are expressed in mathematics.

The only bump in the road is that the {} notation is so similar to 
dictionaries that care must be taken to keep the two as distinct as 
possible.


Raymond