On Aug 22, 12:36 pm, Terry Reedy <[EMAIL PROTECTED]> wrote: > Peter Otten wrote: > > ++imanshu wrote: > >> I agree. Iterator is more flexible. > > I disagree. Neither is more flexible. You can iter the list returned > by sorted and list the iter returned by reversed. Both do the minimum > work necessary. See below. > > > Together and both might have returned the same types. > > True, but only by doing potentially unnecessary work and requiring the > caller to do potentially unnecessary work that might even prevent the > program from working. This is less flexible. > > Suppose sorted now returns alist with 50 million items. Suppose it > instead returned iter(alist) but the caller wants to randomly index the > items. Since the caller could not access the existing 50 million item > list, the caller would have to make another 50 million item copy. This > is non-trivial and might not even work do to memory limitations. > > > It's easy to generate a reversed sequence on the fly but impractical for a > > sorted one. Python is taking the pragmatic approach here. > > To expand on this: sorting and reversing are algorithmically different > operations. Sorting requires that one have all items in hand in a > mutable sequence (list) for arbitrary re-ordering. Sorted works on any > iterable and starts by making a new list. There is no point to not > returning that list after it is sorted. It would be more work and less > useful to do more. > > sorted(iterable, key=None, reverse=False): > newlist = list(iterable) > newlist.sort(key, reverse) > return newlist > > Iterating over a concrete sequence in reverse order, on the other hand, > is trivial. It would be more work and less useful to do more. > > def _reversed(seq): # 'hidden' generator function > n = len(seq) > while n: > n -= 1 > yield seq[n] > > def reversed(seq): > if hasattr(seq, '__reversed__'): > return seq.__reversed__() # I presume this is tried first > else: > return _reversed(seq) # generic fall-back > > Terry Jan Reedy
Thanks for giving the 'behind the scenes' reasons. It looks reasonable now. Thank You, ++imanshu -- http://mail.python.org/mailman/listinfo/python-list
