iterations destroy reversed() results

[Dom Grigonis]

It is by design. `sorted` returns a list, while `reversed` returns an iterator. Iterators are exhaust-able, and not reusable. So be mindful of this and if you are going to "re-use” the sequence returned by iterator, convert it to list first.

Have a look at `itertools` library, which contains a lot of such functions and many good recipes on achieving various things elegantly using iterators.

> On 1 Sep 2023, at 19:15, Pierre Fortin via Python-list <python-list at python.org> wrote:
> 
> Hi,
> 
> reversed() results are fine until iterated over, after which the
> results are no longer available. This was discovered after using
> something like this:
> 
> rev = reversed( sorted( list ) ) 
> sr = sum( 1 for _ in rev )
> # rev is now destroyed
> 
> So reversed() results can only be iterated once unlike sorted(), etc...
> 
> Script to illustrate the issue:
> /tmp/rev:
> orig = [ 'x', 'a', 'y', 'b', 'z', 'c' ]
> co = sum( 1 for _ in orig )
> print( 'orig', orig, co )
> # reversing
> rev = reversed(orig)
> print( 'before iteration:', [ x for x in rev ] )
> # list comprehension was an iteration over 'rev'
> print( 'after iteration:', [ x for x in rev ] )
> # how this was discovered...
> orig = [ 'x', 'a', 'y', 'b', 'z', 'c' ]
> rev = reversed(orig)
> cr = sum( 1 for _ in rev )
> print( 'after sum():', [ x for x in rev ] )
> 
> which produces:
> 
> $ python /tmp/rev
> orig ['x', 'a', 'y', 'b', 'z', 'c'] 6
> before iteration: ['c', 'z', 'b', 'y', 'a', 'x']
> after iteration: []
> after sum(): []
> 
> Regards,
> Pierre
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