[Numpy-discussion] Can't seem to use np.insert() or np.append() for structured arrays

[Sebastian Berg]

On Sa, 2014-08-30 at 09:04 +0100, Sebastian Berg wrote:
> On Fr, 2014-08-29 at 22:10 -0400, Benjamin Root wrote:
> > Consider the following:
> > 
> > a = np.array([(1, 'a'), (2, 'b'), (3, 'c')], dtype=[('foo', 'i'),
> > ('bar', 'a1')])
> > 
> > b = np.append(a, (4, 'd'))
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> >   File
> > "/home/ben/miniconda/lib/python2.7/site-packages/numpy/lib/function_base.py", line 3555, in append
> >     return concatenate((arr, values), axis=axis)
> > TypeError: invalid type promotion
> > b = np.insert(a, 4, (4, 'd'))
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> >   File
> > "/home/ben/miniconda/lib/python2.7/site-packages/numpy/lib/function_base.py", line 3464, in insert
> >     new[slobj] = values
> > ValueError: could not convert string to float: d
> > 
> 

Actually, for insert it is easy to fix
(https://github.com/numpy/numpy/pull/5022), for append there are some
difficulties, because the dtype is not forced to be the arrays dtype,
but gotten from both the original and the appended value currently.

- Sebastian


> Ooops, nice bug in there, might have been me :) (will open a PR). 
> 
> - Sebastian
> 
> > 
> > In my original code snippet I was developing which has a more involved
> > dtype, I actually got a different exception:
> > b = np.append(a, c)
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> >   File
> > "/home/ben/miniconda/lib/python2.7/site-packages/numpy/lib/function_base.py", line 3553, in append
> >     values = ravel(values)
> >   File
> > "/home/ben/miniconda/lib/python2.7/site-packages/numpy/core/fromnumeric.py", line 1367, in ravel
> >     return asarray(a).ravel(order)
> >   File
> > "/home/ben/miniconda/lib/python2.7/site-packages/numpy/core/numeric.py", line 460, in asarray
> >     return array(a, dtype, copy=False, order=order)
> > ValueError: setting an array element with a sequence.
> > 
> > 
> > Luckily, this works as a work-around:
> > >>> b = np.append(a, np.array([(4, 'd')], dtype=a.dtype))
> > >>> b
> > array([(1, 'a'), (2, 'b'), (3, 'c'), (4, 'd')], 
> >       dtype=[('foo', 'i'), ('bar', 'S1')])
> > 
> > 
> > 
> > The same happens whether I enclose the value with square bracket or
> > not. I suspect that this array type just wasn't considered when its
> > checking logic was developed. This is with 1.8.2 from miniconda.
> > Should we consider this a bug or are structured arrays just not
> > expected to be modified like this?
> > 
> > 
> > Cheers!
> > 
> > Ben Root
> > 
> > _______________________________________________
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> > http://mail.scipy.org/mailman/listinfo/numpy-discussion
> 
> 
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