[Python-3000] Set literal

[Larry Hastings]

Nicko van Someren wrote:
> Do we really want set literals at all, given that set(...) exists?
>
> If we are going to have one then, it seems to make sense to have  
> both.  If we are going to have both, I would rather that they generate  
> the same type of set.


Maybe the postings crossed in the ether, but Guido said as much on the 
30th; either they'd both change or neither would change.

Personally I like the idea of changing them to frozensets; a) it's easy 
to cast to set() if you want mutability, b) it makes the

    if x in { 1, 2, 5 }: # three, sir!

idiom faster.  Yeah, it's a bit ticklish wrt dict 
literals/comprehensions returning mutable types, but at least tuple() 
would have company in the immutable constants department.  So +1 from 
me... for what little that is worth.

When compiling a mutable type with an immutable equivalent, does Python 
generate create-empty-object/insert-each-item bytecodes, or does it 
create an immutable constant then cast it to the appropriate type?  My 
intuition is that the latter would be faster; given the professionalism 
of the Python development community, I assume both approaches have been 
tried, and the faster one was switched to long ago?  A coworker of mine 
got a tiny--but measurable--speedup by applying that style of 
optimization to PHP arrays.


/larry/

p.s. or maybe the create-empty-object takes an optional parameter of 
"and reserve this much space inside"?  That would probably even the 
playing field.
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