[Numpy-discussion] Will transpose ever need to copy data?

Jaime Fernández del Río jaime.frio at gmail.com
Wed Jun 12 16:43:54 EDT 2013

On Wed, Jun 12, 2013 at 6:48 AM, Nathaniel Smith <njs at pobox.com> wrote:

> Sounds like a doc bug. (Probably someone being over-careful -- the
> default for many operations in numpy is that it's undefined whether
> they return a view or not, so if it makes a difference to you you need
> to take an explicit copy or similar.) Want to submit a PR to fix the
> doc?

Is it just in the function's docstring that the correction has to be made?

I have also found a potential inconsistency related to this in np.rollaxis,
the following examples should demonstrate it:

>>> a = np.empty((2, 3))
>>> b = np.transpose(a, (0, 1))
>>> b.base is a
>>> c = np.rollaxis(a, 0, 1)
>>> c.base is a
>>> c is a

It seems that np.transpose always returns a view of the passed array, while
np.rollaxis either returns a view or the same array. I personally find this
to be an annoying behavior: similarly to np.nonzero always returning a
tuple, even when it holds a single item, this changing behavior can lead to
hard to track bugs. Once you know what's going on, it is easy to work
around by calling np.rollaxis on a[:] instead of a, but I doubt I will
remember that the next time I need to use it.

This is, by the way, also the behavior of np.ndarray.swapaxes, although it
adds another degree of annoyance to the way it works:

>>> d = a.swapaxes(0, 0)
>>> d.base is a
>>> d is a
>>> e = a.swapaxes(0, -2)
>>> e.base is a

I find that the right thing to do would be to eliminate the checks for
special cases from both np.rollaxis and np.ndarray.swapaxes. The least
would be, in np.ndarray.swapaxes, to move the check for equality of the
axes to be swapped until after negative axis values have been dealt with.
Should any of these changes be included in the doc change PR?

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