Trying to replace part of array to another array, get error ValueError: assignment destination is read-only

Question

I have two arrays with pixels, I need to replace the first part of array 'pixels_new', to array 'pixels_old'

pixels_old = numpy.asarray(im) #picture 100X100
pixels_new = numpy.asarray(img) #picture 100X200

for k in range(0,101):
   for i in range(len(pixels_old[k])):
       print(pixels_new[i])
       print(pixels_old[i])
       pixels_new[i] = pixels_old[i]

It gives me an error:

File "/Users/apple/Desktop/projects/BariySatarov/exercise22.py", line 48, in make_new_im
pixels_new[i] = pixels_old[i]
ValueError: assignment destination is read-only

Please help me with it

Answer 1

I did a simple test as below.

import numpy as np
data = np.asarray([1, 2, 3])
data[0] = 2
data
>>> array([2, 2, 3])

This shows np.asarray does not return the immutable variable, which is read-only. The error is ValueError: assignment destination is read-only. So, I think your im and img variables are immutable. You haven't included the source code for your im and img, so I couldn't tell much about this. However, I have one suggestion is that you should clone or convert your im and img into nd.array before proccessing.

Answer 2

When we see this message :???ValueError: assignment destination is read-only

We need to take two steps to solve it:

First:

print(pixels_new.flags)

output:

C_CONTIGUOUS : True
F_CONTIGUOUS : False
OWNDATA : False
WRITEABLE : False. ------> # this option must be True
ALIGNED : True
WRITEBACKIFCOPY : False
UPDATEIFCOPY : False

then:

pixels_new.flags.writeable = True

The problem will be solved, but if the following message is given, do the following:

ValueError: cannot set WRITEABLE flag to True of this array

Make a copy of it so you can apply the changes to it:

pixel_n = pixels_new.copy()
pixels_n[i] = pixels_old[i]