[Numpy-discussion] Strange inversion in shape with some slices
Travis Oliphant
oliphant@enthought....
Tue Nov 17 13:24:21 CST 2009
On Nov 17, 2009, at 12:33 PM, Sebastian Haase wrote:
> On Tue, Nov 17, 2009 at 5:40 PM, <josef.pktd@gmail.com> wrote:
>> On Tue, Nov 17, 2009 at 11:24 AM, Jean-Baptiste Rudant
>> <boogaloojb@yahoo.fr> wrote:
>>> Hello,
>>> I think there's something strange with shape when a slice is given
>>> by an
>>> array.
>>> import numpy as N
>>> my_array = N.ones((2, 3, 6))
>>> ind = N.arange(4)
>>> #you hope to find (3, 4)
>>> print my_array[0, :, ind].shape
>>> print my_array[0, :, 0:4].shape
>>> print my_array[0][:, ind].shape
>>> print my_array[0][:, 0:4].shape
>>> """
>>> (4, 3)
>>> (3, 4)
>>> (3, 4)
>>> (3, 4)
>>> """
>>> Jean-Baptiste Rudant
>>
>> there was a long thread on this with the explanation in March, title
>> "is it a bug?"
>>
> The thread is here:
> http://www.mail-archive.com/numpy-discussion@scipy.org/msg16300.html
>
> It looks to me like violating the principle of "least surprise".
> My reading of that thread is, that the observed "bug" is mostly a
> consequence coming from the way fancy indexing is implemented. How
> about deprecating this kind of index mixing !?
I don't think this is really a case of the way fancy indexing is
implemented. It could have been done in many ways. The problem is
one of real ambiguity in the general case and the fact that there
wasn't any code written to handle the less ambiguous special-cases
like this one.
There are real applications of index mixing, so turning it off is not
an option. We could perhaps add better handling of circumstances
like this, however, with appropriate warnings and transitions.
-Travis
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