What use for reversed()?

[Jussi Piitulainen]

fl writes:

> On Sunday, May 31, 2015 at 12:59:47 PM UTC-7, Denis McMahon wrote:
>> On Sun, 31 May 2015 12:40:19 -0700, fl wrote:
>> reversed returns an iterator, not a list, so it returns the reversed
>> list of elements one at a time. You can use list() or create a list
>> from reversed and then join the result:

[snip]

> I follow your reply with these trials:
>
>  >>>>list_r=(reversed("fred"))
>  >>> list(list_r)
>  ['d', 'e', 'r', 'f']
>  >>> list_r
>  <reversed object at 0x02B57F10>
>
> I have searched about list, but I still don't know what list_r is.

It's an object that will produce elements on demand. The doc string
(Python 2.7.6) calls it an iterator: "reversed(sequence) -> reverse
iterator over values of the sequence".

Such objects "have state". Try to consume it *twice*, it'll be empty the
second time:

  >>> listr = reversed('fred')
  >>> listr
  <reversed object at 0xb70abc2c>
  >>> list(listr)
  ['d', 'e', 'r', 'f']
  >>> listr
  <reversed object at 0xb70abc2c>
  >>> list(listr)
  []

You can ask for the next element of the iterator:

  >>> listr = reversed('fred')
  >>> next(listr)
  'd'
  >>> list(listr)
  ['e', 'r', 'f']


> What else can it be used besides list(list_r)?

Piecemeal walking using next(list_r), as the source of elements in a for
loop, as a sequence argument to many functions that only need to walk it
once (but they will consume it!).

The suggested ''.join(reversed('fred')) is an example.


> I want to show list_r content. This is possibly an illegal question.

You could do this:

  >>> from __future__ import print_function
  >>> listr = reversed('fred')
  >>> for c in listr: print(c, end = '')
  ... else: print()
  ... 
  derf

But then listr won't have that content, or any content, any more. The
act of looking inside has changed listr :)

Such objects are quite powerful when used properly. The sequence they
produce can be larger than available memory because it need only exist
one element at a time.