[Numpy-discussion] A proposed change to rollaxis() behavior for negative 'start' values

[Charles R Harris]

On Thu, Sep 23, 2010 at 12:20 AM, Ralf Gommers
<ralf.gommers at googlemail.com>wrote:

>
>
> On Wed, Sep 22, 2010 at 4:14 AM, Anne Archibald <
> aarchiba at physics.mcgill.ca> wrote:
>
>> Hi Ken,
>>
>> This is a tricky one. The current behaviour of rollaxis is to remove
>> the requested axis from the list of axes and then insert it before the
>> axis specified. This is exactly how python's list insertion works:
>>
>> In [1]: a = range(10)
>>
>> In [3]: a.insert(-1,'a')
>>
>> In [4]: a
>> Out[4]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 'a', 9]
>>
>> And indeed, there's no clean way to add something to the end of a list
>> using insert (apart from the obvious a.insert(len(a),'b') ). For this
>> you have .append(). Unfortunately numpy's rollaxis, while it agrees
>> with insert in its behaviour, doesn't have a move_axis_to_end. The
>> situation is also somewhat muddied by the fact that rollaxis also
>> removes the axis from the original list of axes, so that the
>> interpretation of index numbers is a little more subtle. But I think
>> your suggested behaviour would be confusing because of the conflict
>> with python's insert. How about allowing the string "end" as an
>> argument to rollaxis to specify that the axis should go at the end?
>>
>
> Allowing "end" is an easy solution, but note that moving an axis to the end
> is already possible:
>
>
> >>> a = np.ones((3,4,5,6))
> >>> np.rollaxis(a, 2, len(a)+1).shape  # roll axis to to last position
> (3, 4, 6, 5)
>
> Not consistent with insert though, there you would use len(a) instead of
> len(a)+1. It's a little ugly, but perhaps just documenting this is no worse
> than allowing a string or adding yet another function.
>
>
It is a common enough operation that it would be nice to have a less
cumbersome way to specify it.

Chuck
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