Python array changes values without reason

Question

In the following code, there is no reason I can see why variable "cleary" would change value, but it does. I have restarted pyCharm multiple times but it keeps happening.

import numpy as np

nPeriods = 48
nGens = 1
cleary = np.zeros((nPeriods,nGens,2))
clearz = np.zeros((nPeriods,nGens))

for ii in range(nPeriods):
    for jj in range(nGens):
        temp = cleary
        temp[ii,jj,:] = 1

What am I doing wrong?

Why do you expect that restarting your editor will change the core behavior of the Python language? — SethMMorton, Sep 29 '16 at 17:37
Duplicate: [List changes unexpectedly after assignment. Why is this and how can I prevent it?](https://stackoverflow.com/questions/2612802/list-changes-unexpectedly-after-assignment-why-is-this-and-how-can-i-prevent-it) — Thomas Weller, Mar 04 '22 at 12:00

score 2 · Answer 1 · edited May 23 '17 at 12:06

2

This is the expected behavior. You passed the reference of the ndarray object the name cleary is pointing to to temp when you did:

temp = cleary

You can avoid modifying cleary by assigning a copy of the array to temp:

temp = cleary.copy()

Read How do I pass a variable by reference? to learn more about the underpinnings of name assignments in Python.

edited May 23 '17 at 12:06

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answered Sep 29 '16 at 17:36

Moses Koledoye

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Python array changes values without reason

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