[Python-3000] Change to class construction?

[Nick Coghlan]

Terry Reedy wrote:
> "Nick Coghlan" <ncoghlan at gmail.com> wrote in message 
> news:468FA01A.6040707 at gmail.com...
> | In py3k it expands to:
> |
> |   def <anon>(outermost):
> |     %0 = []
> |     for x in outermost:
> |       for y in seq2:
> |         %0.append(x*y) # Special opcode, not a normal call
> |     return %0
> |   %n = <anon>(seq1)
> 
> Why not pass both seq1 *and* seq2 to the function so both become locals? 
> The difference of treatment is quite surprising.

The inner iterable expressions can't be evaluated early, as they need to 
be re-evaluated for each pass around the outer loop (or loops). An 
example where the iterable expression for the inner loop refers to the 
iteration variable of the outer loop should make that clear:

.>>> [y for x in range(4) for y in range(x)]
[0, 0, 1, 0, 1, 2]

The advantage of the Py3k approach is that it eliminates the current 
semantic differences between a list comprehension and list() with a 
generator expression argument, while keeping most of the performance 
benefits of the special syntax.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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