On 08/01/2010 03:41 AM, Lawrence D'Oliveiro wrote: > In message <87sk2zhpcj....@dpt-info.u-strasbg.fr>, Alain Ketterlin wrote: > >> Lawrence D'Oliveiro <l...@geek-central.gen.new_zealand> writes: >> >>> V = tuple \ >>> ( >>> x >>> / >>> l >>> for x in V >>> for l in >>> (math.sqrt(reduce(lambda a, b : a + b, (y * y for y in V), >>> 0)),) >>> ) >> >> You got the order wrong (it has to be for l ... for x ...) > > No, I deliberately put it in that order to ensure that the value for l can > only ever be evaulated once. > >> You're kind of lucky here, because the arglist to tuple() provides a >> scope that hides x and l. Be careful if you ever change tuple(...) to >> [...], because x and l would leak to the outer scope (with python 2.*). > > Interesting. However, using “list( ... )” instead of “[ ... ]” also prevents > the leakage.
Yes. That's because the variable leakage of list comprehensions was never a good idea, and is kept in Python 2.x only for backwards compatibility. When generator expressions where introduced, it was done in such a way that the scope didn't leak (which probably wouldn't make any sense anyway since there's no guarantee a generator expression is executed at all before the next line) In Python 3, list comprehensions don't leak names any more - they act (nearly?) the same as like( ... expr ... ) now. -- http://mail.python.org/mailman/listinfo/python-list
