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

[Sebastian Berg]

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
> 

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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