iterators and views of lists

[Carl Banks]

On Dec 17, 10:00 pm, Brendan Miller <catph... at catphive.net> wrote:
> On Thu, Dec 17, 2009 at 6:44 PM, Steven D'Aprano
>
> <st... at remove-this-cybersource.com.au> wrote:
> > On Thu, 17 Dec 2009 12:07:59 -0800, Brendan Miller wrote:
>
> >> I was thinking it would be cool to make python more usable in
> >> programming competitions by giving it its own port of the STL's
> >> algorithm library, which needs something along the lines of C++'s more
> >> powerful iterators.
>
> > For the benefit of those of us who aren't C++ programmers, what do its
> > iterators do that Python's don't?
>
> Python iterators basically only have one operation:
>
> next(), which returns the next element or throws StopIteration.
>
> In C++ terminology this is a Input iterator. It is good for writing
> "for each" loops or map reduce operations.

Hmm.  I guess the main thing that's bothering me about this whole
thread is that the true power of Python iterators is being overlooked
here, and I don't think you're being fair to call Python iterators
"weak" (as you did in another thread) or say that they are only good
for for-else type loops.

The fact is, Python iterators have a whole range of powers that C++
iterators do not.  C++ iterators, at least the ones that come in STL,
are limited to iterating over pre-existing data structures.  Python
iterators don't have this limation--the data being "iterated" over can
be virtual data like an infinite list, or data generated on the fly.
This can be very powerful.

Here's a cool slideshow on what can be done with iterators in Python
(generators specifically):

http://www.dabeaz.com/generators/

It is true that Python iterators can't be used to mutate the
underlying structure--if there is actual underlying data structure--
but it doesn't mean they are weak or limited.  Python and C++
iterators are similar in their most basic usage, but grow more
powerful in different directions.



Carl Banks