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
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