[Numpy-discussion] Suggestion: special-case np.array(range(...)) to be faster

[Antony Lee]

In a sense this discussion is really about making np.array(iterable) more
efficient, so I restarted the discussion at
https://mail.scipy.org/pipermail/numpy-discussion/2016-February/075059.html

Antony

2016-02-18 14:21 GMT-08:00 Chris Barker <chris.barker at noaa.gov>:

> On Thu, Feb 18, 2016 at 10:15 AM, Antony Lee <antony.lee at berkeley.edu>
> wrote:
>
>> Mostly so that there is no performance lost when someone passes
>> range(...) instead of np.arange(...).  At least I had never realized that
>> one is much faster than the other and always just passed range() as a
>> convenience.
>>
>
> Well,  pretty much everything in numpy is faster if you use the numpy
> array version rather than plain python -- this hardly seems like the extra
> code would be worth it.
>
> numpy's array() constructor can (and should) take an arbitrary iterable.
>
> It does make some sense that you we might want to special case iterators,
> as you don't want to loop through them too many times, which is what
> np.fromiter() is for.
>
> and _maybe_ it would be worth special casing python lists, as you can
> access items faster, and they are really, really common (or has this
> already been done?), but special casing range() is getting silly. And it
> might be hard to do. At the C level I suppose you could actually know what
> the parameters and state of the range object are and create an array
> directly from that -- but that's what arange is for...
>
> -CHB
>
>
>
>> 2016-02-17 10:50 GMT-08:00 Chris Barker <chris.barker at noaa.gov>:
>>
>>> On Sun, Feb 14, 2016 at 11:41 PM, Antony Lee <antony.lee at berkeley.edu>
>>> wrote:
>>>
>>>> So how can np.array(range(...)) even work?
>>>>
>>>
>>> range()  (in py3) is not a generator, nor is is a iterator. it is a
>>> range object, which is lazily evaluated, and satisfies both the iterator
>>> protocol and the sequence protocol (at least most of it:
>>>
>>> In [*1*]: r = range(10)
>>>
>>>
>>> In [*2*]: r[3]
>>>
>>> Out[*2*]: 3
>>>
>>>
>>> In [*3*]: len(r)
>>>
>>> Out[*3*]: 10
>>>
>>>
>>> In [*4*]: type(r)
>>>
>>> Out[*4*]: range
>>>
>>> In [*9*]: isinstance(r, collections.abc.Sequence)
>>>
>>> Out[*9*]: True
>>>
>>> In [*10*]: l = list()
>>>
>>> In [*11*]: isinstance(l, collections.abc.Sequence)
>>>
>>> Out[*11*]: True
>>>
>>> In [*12*]: isinstance(r, collections.abc.Iterable)
>>>
>>> Out[*12*]: True
>>> I'm still totally confused as to why we'd need to special-case range
>>> when we have arange().
>>>
>>> -CHB
>>>
>>>
>>>
>>> --
>>>
>>> Christopher Barker, Ph.D.
>>> Oceanographer
>>>
>>> Emergency Response Division
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>>>
>>> Chris.Barker at noaa.gov
>>>
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>>>
>>
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>
>
> --
>
> Christopher Barker, Ph.D.
> Oceanographer
>
> Emergency Response Division
> NOAA/NOS/OR&R            (206) 526-6959   voice
> 7600 Sand Point Way NE   (206) 526-6329   fax
> Seattle, WA  98115       (206) 526-6317   main reception
>
> Chris.Barker at noaa.gov
>
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