[Numpy-discussion] 0-dimensional object arrays

Ed Schofield schofield at ftw.at
Tue Jan 24 06:16:02 CST 2006

Hi all,

I've been attempting to use an object array to group together a
collection of different-length arrays or lists, and I'm getting
unexpected results.  Here's an example:

>>> import numpy
>>> a = numpy.array([[1,2],[1,2,3]],object)

I expected here that 'a' would be an array of dimension (2,), holding
two Python lists of integers.  Was I wrong to expect this?  It seems
instead that 'a' is a 0-dimensional array holding one object:

>>> a
array([[1, 2], [1, 2, 3]], dtype=object)
>>> a.shape
>>> type(a)
<type 'numpy.ndarray'>
>>> a[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
IndexError: 0-d arrays can't be indexed.

This behaviour seems less useful.  Would you agree this is a bug in the
array constructor?

I also can't explain this behaviour:

>>> [] + a
array([], dtype=object)
>>> a + []
array([], dtype=object)

I guess I would have expected these commands to be equivalent

These results seem more sensible:

>>> a.transpose()
[[1, 2], [1, 2, 3]]
>>> a*2
[[1, 2], [1, 2, 3], [1, 2], [1, 2, 3]]

although these last two commands return Python lists, without the 0-d
array wrapper, which isn't consistent with other 0-d arrays.

On the whole, 0-d object arrays seem quite strange beasts.  Could
someone please enlighten me on why they deserve to exist? ;)  They seem
inconsistent with the new simplified interface to object array
elements.  Could we get rid of them entirely?!

-- Ed

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