[Numpy-discussion] possible bug with numpy.object_

Fernando Perez fperez.net at gmail.com
Thu Aug 31 10:08:36 CDT 2006


On 8/31/06, Travis Oliphant <oliphant.travis at ieee.org> wrote:

> What about
>
> N.array(3).size
>
> N.array([3]).size
>
> N.array([3,3]).size
>
> Essentially, the [] is being treated as an object when you explicitly
> ask for an object array in exactly the same way as 3 is being treated as
> a number in the default case.  It's just that '[' ']' is "also" being
> used as the dimension delimiter and thus the confusion.
>
> It is consistent.   It's a corner case, and I have no problem fixing the
> special-case code running when dtype=object so that array([],
> dtype=object) returns an empty array, if that is the consensus.

I wasn't really complaining: these are corner cases I've never seen in
real use, so I'm not really sure how critical it is to worry about
them.  Though I could see code which does automatic size/shape checks
tripping on some of them.  The shape tuples shed a bit of light on
what's going on for the surprised (like myself):

In [8]: N.array(3).shape
Out[8]: ()

In [9]: N.array([3]).shape
Out[9]: (1,)

In [10]: N.array([3,3]).shape
Out[10]: (2,)

In [11]: N.array([]).shape
Out[11]: (0,)

In [12]: N.array([[]]).shape
Out[12]: (1, 0)

In [13]: N.array([[],[]]).shape
Out[13]: (2, 0)


I won't really vote for any changes one way or another, as far as I'm
concerned it's one of those 'learn the library' things.  I do realize
that the near-ambiguity between '[]' as an empty object and '[]' as
the syntactic delimiter for a container makes this case a bit of a
gotcha.

I guess my only remaining question is: what is the difference between
outputs #8 and #11 above?  Is an empty shape tuple == array scalar,
while a (0,) shape indicates a one-dimensional array with no elements?
 If this interpretation is correct, what is the usage of the latter
kind of object, given how it can't even be indexed?

In [15]: N.array([])[0]
---------------------------------------------------------------------------
exceptions.IndexError                                Traceback (most
recent call last)

/home/fperez/research/code/mjmdim/pycode/<ipython console>

IndexError: index out of bounds


And is this really expected?

In [18]: N.array([]).any()
Out[18]: False

In [19]: N.array([]).all()
Out[19]: True

It's a bit funny to have an array for which 'no elements are true'
(any==false), yet 'all are true' (all==true), isn't it?

Regards,

f




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