# Sorted Returns List and Reversed Returns Iterator

Discussion in 'Python' started by ++imanshu, Aug 22, 2008.

1. ### ++imanshuGuest

Hi,

Is there a reason why two similarly named functions Sorted and
Reversed return different types of data or is it an accident.

Thanks,
++imanshu

++imanshu, Aug 22, 2008

2. ### John MachinGuest

On Aug 22, 12:12 pm, "++imanshu" <> wrote:
> Hi,
>
>      Is there a reason why two similarly named functions Sorted and
> Reversed return different types of data or is it an accident.

You seem to have an interesting notion of "similarly named".
name0[-2:] == name1[-2:], perhaps? The two functions (eventually, in
the case of "reversed") return data in the order one would expect from
their names.

>>> x = [1, 3, 5, 2, 4, 6]
>>> sorted(x)

[1, 2, 3, 4, 5, 6]
>>> reversed(x)

<listreverseiterator object at 0x00AA5550>
>>> list(reversed(x))

[6, 4, 2, 5, 3, 1]
>>>

John Machin, Aug 22, 2008

3. ### John MachinGuest

On Aug 22, 1:35 pm, John Machin <> wrote:
> On Aug 22, 12:12 pm, "++imanshu" <> wrote:
>
> > Hi,

>
> >      Is there a reason why two similarly named functions Sorted and
> > Reversed return different types of data or is it an accident.

>
> You seem to have an interesting notion of "similarly named".
> name0[-2:] == name1[-2:], perhaps? The two functions (eventually, in
> the case of "reversed") return data in the order one would expect from
> their names.
>
> >>> x = [1, 3, 5, 2, 4, 6]
> >>> sorted(x)

> [1, 2, 3, 4, 5, 6]
> >>> reversed(x)

>
> <listreverseiterator object at 0x00AA5550>
>
>
>
> >>> list(reversed(x))

> [6, 4, 2, 5, 3, 1]-

Sorry; having re-read the message subject:

reversed came later; returning an iterator rather than a list provides
more flexibility.

Cheers,
John

John Machin, Aug 22, 2008
4. ### ++imanshuGuest

On Aug 22, 8:40 am, John Machin <> wrote:
> On Aug 22, 1:35 pm, John Machin <> wrote:
>
>
>
> > On Aug 22, 12:12 pm, "++imanshu" <> wrote:

>
> > > Hi,

>
> > >      Is there a reason why two similarly named functions Sorted and
> > > Reversed return different types of data or is it an accident.

>
> > You seem to have an interesting notion of "similarly named".
> > name0[-2:] == name1[-2:], perhaps? The two functions (eventually, in
> > the case of "reversed") return data in the order one would expect from
> > their names.

>
> > >>> x = [1, 3, 5, 2, 4, 6]
> > >>> sorted(x)

> > [1, 2, 3, 4, 5, 6]
> > >>> reversed(x)

>
> > <listreverseiterator object at 0x00AA5550>

>
> > >>> list(reversed(x))

> > [6, 4, 2, 5, 3, 1]-

>
> Sorry; having re-read the message subject:
>
> reversed came later; returning an iterator rather than a list provides
> more flexibility.
>
> Cheers,
> John

I agree. Iterator is more flexible. Together and both might have
returned the same types.

Thanks,
++imanshu

++imanshu, Aug 22, 2008
5. ### Peter OttenGuest

++imanshu wrote:

> On Aug 22, 8:40Â am, John Machin <> wrote:
>> On Aug 22, 1:35Â pm, John Machin <> wrote:
>>
>>
>>
>> > On Aug 22, 12:12Â pm, "++imanshu" <> wrote:

>>
>> > > Hi,

>>
>> > > Is there a reason why two similarly named functions Sorted and
>> > > Reversed return different types of data or is it an accident.

>>
>> > You seem to have an interesting notion of "similarly named".
>> > name0[-2:] == name1[-2:], perhaps? The two functions (eventually, in
>> > the case of "reversed") return data in the order one would expect from
>> > their names.

>>
>> > >>> x = [1, 3, 5, 2, 4, 6]
>> > >>> sorted(x)
>> > [1, 2, 3, 4, 5, 6]
>> > >>> reversed(x)

>>
>> > <listreverseiterator object at 0x00AA5550>

>>
>> > >>> list(reversed(x))
>> > [6, 4, 2, 5, 3, 1]-

>>
>> Sorry; having re-read the message subject:
>>
>> reversed came later; returning an iterator rather than a list provides
>> more flexibility.
>>
>> Cheers,
>> John

>
> I agree. Iterator is more flexible. Together and both might have
> returned the same types.

It's easy to generate a reversed sequence on the fly but impractical for a
sorted one. Python is taking the pragmatic approach here.

Peter

Peter Otten, Aug 22, 2008
6. ### Fredrik LundhGuest

John Machin wrote:

> reversed came later; returning an iterator rather than a list provides
> more flexibility.

As in flexibility for the implementer, the day someone invents a sort
algorithm that doesn't have to look at all source items before it starts
producing output?

Because I fail to see how returning an object that supports forward
iteration only is more flexible for the *user* than returning an object
that supports random access, mutation, restartable iteration, *and*
forward iteration.

</F>

Fredrik Lundh, Aug 22, 2008
7. ### Terry ReedyGuest

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

Terry Reedy, Aug 22, 2008
8. ### ++imanshuGuest

On Aug 22, 12:36 pm, Terry Reedy <> 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

++imanshu, Aug 23, 2008