Python error: ValueError: setting an array element with a sequence

Question

I'm doing some image detection. The detection algorithm has a line of code that's causing an error.

The line of code:

im_rois = im_rois.astype(np.float, copy=False)

The error is:

Error: ValueError: setting an array element with a sequence.

I printed the contents of im_rois. They are:

[[ array([[  56.04,   57.32,  317.16,  285.16],
       [ 125.16,   17.64,  332.52,  285.16],
       [   1.  ,    1.  ,  427.24,  639.72],
       ..., 
       [ 201.96,    1.  ,  248.04,   50.92],
       [ 286.44,    1.  ,  350.44,   47.08],
       [ 304.36,  244.2 ,  338.92,  349.16]])]]

Curious if anyone can point me in the direction.

You don't seems to have a array but a list of list of array!! You can try im_rois = im_rois[0][0]. — Clodion
– Clodion, Commented Jul 26, 2015 at 15:14
I think it's best to ask the author of im_rois.astype. Either to provide more meaningful error, or debug their code. — luk32
– luk32, Commented Jul 26, 2015 at 15:15
Are you sure you haven't passed in an invalid argument to a function in the code you are using? The name im_rois suggests a sequence of "regions of interest", which (maybe) should be an array with shape (N, 4), where N is the number of RoIs. It looks like your im_rois is nested too deep. — Warren Weckesser
– Warren Weckesser, Commented Jul 26, 2015 at 15:27
Thanks, yes its quite possible I've got some invalid inputs. The roi inputs are generated by another algorithm called selective search using matlab. The matlab file is loaded and converted into a roi array. I'm trying to figure out if I should go back and fix my matlab based code or persist with the matlab output I have. — gobob
– gobob, Commented Jul 26, 2015 at 15:33
Is this matlab file a .mat? Do you load it with scipy loadmat? Matlab matrices load as numpy arrays, but matlab cells load as python lists (but may contains arrays). — hpaulj
– hpaulj, Commented Jul 27, 2015 at 16:53

hpaulj · Accepted Answer · 2015-07-27 04:25:38Z

I can reproduce your error with this sequence:

In [1317]: im=[[np.array([[1,2,3]])]]  # list containing array

im.astype(float) would produce a AttributeError: 'list' object has no attribute 'astype' because it is a list, not an array.

But I can embed it in an object array:

In [1318]: x=np.empty((1,),dtype=object)
In [1319]: x[0]=im    
In [1320]: x
Out[1320]: array([[[array([[1, 2, 3]])]]], dtype=object)

The print looks like yours; note the commas in the inner list.

In [1321]: print(x)
[[[array([[1, 2, 3]])]]]

If it was a ndim array, the print wouldn't have the commas.

In [1325]: print(np.array([[[1,2,3]]]))
[[[1 2 3]]]

Now when I try a astype I get your value error.

In [1322]: x.astype(np.float)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1322-1bc194707569> in <module>()
----> 1 x.astype(np.float)

ValueError: setting an array element with a sequence.

But without knowing how im_rois is created I can't suggest ways of avoiding this. If im_rois were

array([[  56.04,   57.32,  317.16,  285.16],
       [ 125.16,   17.64,  332.52,  285.16],
       [   1.  ,    1.  ,  427.24,  639.72],
       ...]])

it wouldn't give this error.

I'd check to make you sure you are feeding this package the right kinds of input, whether it be a list, an array or a scalar.

vstack is able to remove this intermediate 'object' layer, allowing astype.

In [1343]: np.vstack(x)
Out[1343]: array([[[[1, 2, 3]]]])

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