[Numpy-discussion] Can't mix np.newaxis with boolean indexing

Benjamin Root ben.root@ou....
Sat Aug 20 15:17:02 CDT 2011


On Sat, Aug 20, 2011 at 2:47 AM, Olivier Verdier <zelbier@gmail.com> wrote:

> Your syntax is not as intuitive as you may think.
>
> Suppose I take a matrix instead
>
> a = np.array([1,2,3,4]).reshape(2,2)
> b = (a>1) # np.array([[False,True],[True,True]])
>
> How would a[b,np.newaxis] be supposed to work?
>
> Note that other (simple) slices work perfectly with newaxis, such as
> a[:1,np.newaxis]
>
> == Olivier
>
>
Personally, I would have expected it to flatten the results and added a
dimension:

a[b, np.newaxis]
array([[2],
         [3],
         [4]])

or

a[np.newaxis, b]
array([[2, 3, 4]])

I mean, it flattens the results anyway when doing boolean indexing for
multi-dimensional arrays, so someone doing that should expect that anyway.

At the very least, I think maybe we could have a better error message than
just saying that long() can't take a NoneType?

Thanks,
Ben Root
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