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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: I am out of trial and error again Lists
Date: Thu, 23 Oct 2014 23:14:37 -0400
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On 10/23/2014 3:39 PM, Ian Kelly wrote:
> On Thu, Oct 23, 2014 at 11:07 AM, Seymore4Head
> <Seymore4Head@hotmail.invalid> wrote:
>> BTW I forgot to add that example 2 and 3 don't seem to be too useful
>> in Python 3, but they are in Python 2.  I don't understand how the
>> Python 3 is an improved version.
>
> In Python 2, range returns a list containing all the requested
> elements. This is simple to work with but often not desirable, because
> it forces the entire sequence to be loaded into memory at once. Large
> ranges may cause excessive memory use and related slowdowns, while
> very large ranges may not even be usable.
>
> In Python 3, range returns a compact object that knows what elements
> it contains and can iterate over them (which is the most common use of
> ranges by far), but without needing to load them all into memory at
> once. While iterating, only the current element needs to exist in
> memory. You can still get the range as a list if you want it, by
> calling list(range(10)), but it doesn't force you to create a list,
> which makes it more versatile than the Python 2 construct.
>
> The more specialized data structure used in Python 3 also allows for
> certain optimizations, for example membership testing. In Python 2, if
> you do the test "97 in range(100)", it has to construct a 100-element
> list and then iterate over 97 of the elements before discovering that
> the list does in fact contain the number 97. The Python 3 range object
> knows what the bounds of the range are, so all it has to do is check
> that 0 <= 97 < 100 to know that the number 97 is included in the
> range.

Example that works 'instantly' in 3.x, but would be slightly insane in 
2.x, even if you have enough memory for it to work on a 64 bit build.

 >>> r =range(0, 1000000000000000, 999937)
 >>> 3428761974385 in r
False
 >>> 3428761974386 in r
True

-- 
Terry Jan Reedy