Python - 在 For 循环内调用函数 - 更改输入参数而不覆盖它

Question

Novice Python user here, really stumped on this one. I have a 3x3 array that stores coordinates in xyz format, where the rows are the number of atoms and columns correspond to x,y and z. For every element that is not in the z direction I wish to add some scalar dr to it. Ultimately I would like to generate a dictionary of 6 geometries, where in each instance one of the elements x0,y0 x0,y1, x1,y0 etc. have the scalar added. For now I am just trying to write a function to do this and print out the geometries at each iteration.

This is a simpler version of the function I have written. Here I loop over the rows and columns, call a the function with the arguments of reference geometry (geom), and the two indexes for the rows and columns. For each X and Y coordinate the function then adds dr to geometry and returns its value.

import numpy as np

dr = 0.1
principle_axes = 'Z'


def displace(coords, row, col):
    if principle_axes == 'Z':
        if col != 2:
            new_coords = coords
            new_coords[row, col] = new_coords[row, col] + dr
            return new_coords


geom = np.array([[0, 0, 0.1435], [0, 0, 2.992], [0, 0, -2.8993]])
[nR, nC] = np.shape(geom)
if nR == 3 and nC == 3:
    import pdb; pdb.set_trace()  # XXX BREAKPOINT
    for i in range(nR):
        for j in range(nC):
            print(geom)
            displaced_geom = displace(geom, i, j)
            print(displaced_geom)

With each iteration of the loop the function takes the returned geometry value of the last iteration, even though the argument (geom) that is called does not get reassigned during the loop. Which gives me this example output for geom...

[[ 0.      0.      0.1435]
 [ 0.      0.      2.992 ]
 [ 0.      0.     -2.8993]]
[[ 0.1     0.      0.1435]
 [ 0.      0.      2.992 ]
 [ 0.      0.     -2.8993]]
[[ 0.1     0.1     0.1435]
 [ 0.      0.      2.992 ]
 [ 0.      0.     -2.8993]]

The output that results from printing displaced_geom is the same. I wish the resulting output to be:

[[ 0.      0.      0.1435]
 [ 0.      0.      2.992 ]
 [ 0.      0.     -2.8993]]
[[ 0.1     0.      0.1435]
 [ 0.      0.      2.992 ]
 [ 0.      0.     -2.8993]]
[[ 0.     0.1     0.1435]
 [ 0.      0.      2.992 ]
 [ 0.      0.     -2.8993]]
[[ 0.      0.     0.1435]
 [ 0.1      0.      2.992 ]
 [ 0.      0.     -2.8993]]

I can then figure out how to store the result from each iteration in a dictionary and use that to do stuff later on in my code. FYI I am running Python3.5.2 on Ubuntu 16.04.6 LTS. If anyone could help me understand where I have gone wrong and point me in the right direction that would be great.

Answer 1

def displace(coords, row, col):
    if principle_axes == 'Z':
        if col != 2:
            new_coords = coords
            new_coords[row, col] = new_coords[row, col] + dr
            return new_coords

new_coords = coords assigns a pointer, not a copy.

You can instead do new_coords = coords.copy() so that you don't overwrite.