Create a copy and not a reference of a NumPy array

Question

I'm currently working on a Python program that involves using NumPy for image processing. However, I've encountered an issue when trying to create a deep copy of a NumPy array instead of just copying the reference.

Here's a snippet of my code, where I read in a PNG image and assign it to imageOriginal_3d:

width, height, pngData, metaData = png.Reader(file).asDirect()
planeCount = metaData['planes']
print('Image Size: ' + str(width) + 'x' + str(height) + ' Pixel')
image_2d = np.vstack(list(map(np.uint8, pngData)))
imageOriginal_3d = np.reshape(image_2d, (width, height, planeCount)) 
imageEdited_3d = imageOriginal_3d // TODO: CREATE DEEP COPY

My intention is to edit imageEdited_3d without affecting the values in imageOriginal_3d. However, when I modify imageEdited_3d, the changes currently also appear in imageOriginal_3d.

Moinuddin Quadri · Accepted Answer · 2016-10-22 21:16:15Z

13

You need to create the copy of the object. You may do it using numpy.copy() since you are having numpy object. Hence, your initialisation should be like:

imageEdited_3d = imageOriginal_3d.copy()

Also there is copy module for creating the deep copy OR, shallow copy. This works independent of object type. For example, your code using copy should be as:

from copy import copy, deepcopy

# Creates shallow copy of object
imageEdited_3d = copy(imageOriginal_3d)

# Creates deep copy of object
imageEdited_3d = deepcopy(imageOriginal_3d)

Description:

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

edited Oct 22, 2016 at 21:16

answered Oct 22, 2016 at 21:05

Moinuddin Quadri

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2 Comments

monolith Over a year ago

Is there a notable difference in time consumption when constructing a deep copy versus shallow copy?

taper Over a year ago

@wedran There is indeed. Deepcopy is much more time consuming! Try this gist

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