Re: What is xrange?
Dennis Lee Bieber wrote: On Sat, 30 Jul 2011 21:36:41 +1000, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info declaimed the following in gmane.comp.python.general: Probably because nobody thought about it, but if it had been proposed to change xrange into an iterator, I'm pretty confident that the suggestion would have been rejected. xrange objects might be lazily generated, but they're also sequence types: you can get their length, and you can index them. (However you can't slice them.) Iterators are not sequence types: they aren't indexable and you can't get their length. Ah, but what did they change range() into -- I seem to recall reading the Python 3.x turned the regular range() into something else... (from Python 3.x returning a full list) From the What's New docs, in the Views And Iterators Instead Of Lists section: range() now behaves like xrange() used to behave, except it works with values of arbitrary size. The latter no longer exists. ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
On Jul 29, 2011, at 9:22 PM, Steven D'Aprano wrote: Billy Mays wrote: Is xrange not a generator? I know it doesn't return a tuple or list, so what exactly is it? xrange pre-dates generators by approximately forever. It returns a special xrange object, which generates values suitable for use in for-loops using the old __getitem__ protocol. interesting...I never really thought about this. Is there a reason that it wasn't reimplemented when generators came out? Is there a use-case for the current implementation that wouldn't work as a generator? bb -- Brian Blais bbl...@bryant.edu http://web.bryant.edu/~bblais http://bblais.blogspot.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
Brian Blais wrote: On Jul 29, 2011, at 9:22 PM, Steven D'Aprano wrote: Billy Mays wrote: Is xrange not a generator? I know it doesn't return a tuple or list, so what exactly is it? xrange pre-dates generators by approximately forever. It returns a special xrange object, which generates values suitable for use in for-loops using the old __getitem__ protocol. interesting...I never really thought about this. Is there a reason that it wasn't reimplemented when generators came out? Is there a use-case for the current implementation that wouldn't work as a generator? It certainly couldn't be re-implemented before Python 3, because that would change the behaviour and therefore break people's code that expected to be able to treat xrange objects as sequences. And now that Python 3.2 is out, and 3.3 is being worked on, it can't be changed now either, for the same reason. There was a very narrow window of opportunity for the functionality to be changed, and it was missed. Probably because nobody thought about it, but if it had been proposed to change xrange into an iterator, I'm pretty confident that the suggestion would have been rejected. xrange objects might be lazily generated, but they're also sequence types: you can get their length, and you can index them. (However you can't slice them.) Iterators are not sequence types: they aren't indexable and you can't get their length. So why bother *taking away* functionality from xrange just to make it a less powerful and more restricted iterator? It works fine just the way it is, there's no need to change it. It isn't like the Python developers are sitting around bored, looking for things to do. They are overworked with far too much to do and not enough manpower or time to do it. There are a huge number of much more important bug fixes and features that haven't been dealt with for them to bother with something like this. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
On Jul 30, 2011, at 7:36 AM, Steven D'Aprano wrote: xrange objects might be lazily generated, but they're also sequence types: you can get their length, and you can index them. (However you can't slice them.) Iterators are not sequence types: they aren't indexable and you can't get their length. ah! now that makes sense. I guess I never check their length, or index them, and only use them like generators. :) thanks for the clarification! It isn't like the Python developers are sitting around bored, looking for things to do. They are overworked with far too much to do and not enough manpower or time to do it. There are a huge number of much more important bug fixes and features that haven't been dealt with for them to bother with something like this. Now, that seems a little harsh, and nothing that I was intending. I figured (before learning about getting its length and indexing) that if the xrange object was basically a pre-generator hack, that it would make sense from a code-clarity point of view to reimplement them as generators -- it would gave made the developer's life easier. Now that I realize that xrange has different functionality than a generator (which was the point of my question...just curious *what* different functionality) it clearly doesn't make sense to implement them as generators. I certainly wasn't trying to imply that the developers are lazy, or bored! bb -- Steven -- http://mail.python.org/mailman/listinfo/python-list -- Brian Blais bbl...@bryant.edu http://web.bryant.edu/~bblais http://bblais.blogspot.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
On Sun, Jul 31, 2011 at 1:06 AM, Dennis Lee Bieber wlfr...@ix.netcom.com wrote: Ah, but what did they change range() into -- I seem to recall reading the Python 3.x turned the regular range() into something else... (from Python 3.x returning a full list) xrange got renamed to range, as I understand it. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
Brian Blais wrote: On Jul 29, 2011, at 9:22 PM, Steven D'Aprano wrote: Billy Mays wrote: Is xrange not a generator? I know it doesn't return a tuple or list, so what exactly is it? It's an iterable object. There are advantages in having xrange (or range in python3) return an iterable object, rather than creating an iterator directly. One of them is that you can iterate over the same object more than once. This can be useful if you're iterating over a multi-dimensional space of some kind. Some efficiency can be gained by pre-creating an xrange object for each of the inner loops and re-using them. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
What is xrange?
Is xrange not a generator? I know it doesn't return a tuple or list, so what exactly is it? Y doesn't ever complete, but x does. x = (i for i in range(10)) y = xrange(10) print ===X=== while True: for i in x: print i break else: break print ===Y=== while True: for i in y: print i break else: break -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
Billy Mays wrote: Is xrange not a generator? I know it doesn't return a tuple or list, so what exactly is it? Y doesn't ever complete, but x does. range(n) creates a list containing all the integers 0..n-1. This is a problem if you do range(100), because you'll end up with a 4Mb list. xrange deals with this by returning an object that pretends to be a list, but just works out the number needed from the index asked for, and returns that. range() can actually be faster in some cases - eg. if iterating over the same sequence multiple times. xrange has to reconstruct the integer object every time, but range will have real integer objects. (It will always perform worse in terms of memory however) xrange isn't usable in all cases where a real list is needed. For instance, it doesn't support slices, or any list methods. -- m harris FSF ...free as in freedom/ http://webpages.charter.net/harrismh777/gnulinux/gnulinux.htm -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
On 29/07/11 21:36, Billy Mays wrote: Is xrange not a generator? I know it doesn't return a tuple or list, so what exactly is it? Y doesn't ever complete, but x does. x = (i for i in range(10)) y = xrange(10) print ===X=== while True: for i in x: print i break else: break print ===Y=== while True: for i in y: print i break else: break Every for loop calls gets a new iterator from the object you're iterating over. (__iter__) -- Apparently, xrange is implemented in such a way (as are lists) that you can iterate over the object many times, while each generator object (and how could it be otherwise can only be iterated over once. What is xrange(foo)? It is an object that supports list-like indices, the iterator protocol, and probably a few other things that you will find in the stdlib docs. Generators also support the iterator protocol, but that's about as far as the similarity goes (in general) - Thomas -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
On Fri, Jul 29, 2011 at 3:36 PM, Billy Mays no...@nohow.com wrote: Is xrange not a generator? I know it doesn't return a tuple or list, so what exactly is it? Y doesn't ever complete, but x does. x = (i for i in range(10)) y = xrange(10) xrange() does not return a generator. It returns an iterable xrange object. If you want the iterator derived from the iterable xrange object, you can get it like this: iterator = y.__iter__() See http://docs.python.org/library/functions.html#xrange for the definition of the xrange object. http://www.learningpython.com/2009/02/23/iterators-iterables-and-generators-oh-my/ seems to cover the differences between iterables, iterators, and generators pretty well. Some more reading: http://docs.python.org/howto/functional.html http://www.python.org/dev/peps/pep-0255/ http://www.python.org/dev/peps/pep-0289/ -- Jerry -- http://mail.python.org/mailman/listinfo/python-list
Re: What is xrange?
Billy Mays wrote:
Is xrange not a generator? I know it doesn't return a tuple or list, so
what exactly is it?
xrange pre-dates generators by approximately forever. It returns a
special xrange object, which generates values suitable for use in
for-loops using the old __getitem__ protocol.
You can consider xrange to be implemented something vaguely like this:
class My_xrange:
def __init__(self, start, end=None, step=1):
if end is None:
end = start
start = 0
self.start = start
self.end = end
self.step = step
def __getitem__(self, index):
if self.step != 1:
raise NotImplementedError('too lazy to support step values')
start, end = self.start, self.end
if start = index end:
return start + index
raise IndexError('out of range')
Y doesn't ever complete, but x does.
x = (i for i in range(10))
That is better written as iter(range(10)).
y = xrange(10)
Y doesn't complete because xrange objects are restartable. The for-loop ends
up using the original iteration protocol:
i = y[0]
i = y[1]
i = y[2]
...
until IndexError is raised. You break after calling y[0], but the next loop
starts at y[0] again, and so you never progress beyond the first item in
the xrange object.
--
Steven
--
http://mail.python.org/mailman/listinfo/python-list
