[Tutor] list semantics

[Steven D'Aprano]

On Sat, Apr 11, 2015 at 02:15:49PM -0400, Joel Goldstick wrote:
> On Sat, Apr 11, 2015 at 1:41 PM, Jim Mooney <cybervigilante at gmail.com> wrote:
> > Why does the first range convert to a list, but not the second?
> >
> >>>> p = list(range(1,20)), (range(40,59))
> >>>> p
> > ([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
> >  range(40, 59))
> >
> Assuming you are using python 3.x range is a generator, so it only
> produces values if you iterate over it

Almost correct, but not quite. range, like xrange in Python 2, is not a 
generator, but a custom-made lazy sequence object.

py> gen()  # This actually is a generator.
<generator object gen at 0xb7bd7914>
py> range(1, 10)  # This is not.
range(1, 10)


Range objects are special: not only do they produce values lazily as 
needed, but they also support len(), indexing, slicing, and membership 
testing, none of which generators are capable of doing:

py> r = range(1, 30, 3)
py> len(r)
10
py> r[8]  # indexing
25
py> r[2:5]  # slicing
range(7, 16, 3)
py> 9 in r  # membership testing
False
py> 10 in r
True

All those results are calculated lazily, without having to pre-calculate 
a potentially enormous list.


-- 
Steve