ordered sets operations on lists..

[Steve Holden]

Alex Martelli wrote:
> Bengt Richter <bokr at oz.net> wrote:
>    ...
> 
>>>Personally, I'd always use (depending on guesses regarding lengths of
>>>lists) [x for x in l1 if x in l2] or the setified equivalent, of course.
>>>
>>
>>Perhaps newbies should be advised that
>>
>>    [x for x in l1 if x in set(l2)]
>>
>>is not a (well) setified equivalent? I could see them being tempted.
> 
> 
> You mean, newbies should be advised that Python does NOT hoist any
> computations whatsoever from the body of a loop (including LCs and
> genexps), so if you want anything hoisted you need to hoist it yourself?
> Yes, it is a point worth making, since the lack of hoisting is a
> frequent cause of performance loss.
> 
Of course now 2.5 is planning to include the AST parser there's fruitful 
ground for optimization studies that will perform hoisting automatically 
(should anyone be looking for a project ;-)

Given that Python 2.4 doesn't even perform simple constant folding for 
arithmetic expressions

  >>> dis.dis(compile("print 1+2", '', 'exec'))
   1           0 LOAD_CONST               0 (1)
               3 LOAD_CONST               1 (2)
               6 BINARY_ADD
               7 PRINT_ITEM
               8 PRINT_NEWLINE
               9 LOAD_CONST               2 (None)
              12 RETURN_VALUE

even hoisting could be seen as an "advanced" optimization.

regards
  Steve
-- 
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