Python array changes values without reason [duplicate]

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
– SethMMorton, Commented Sep 29, 2016 at 17:37
Duplicate: List changes unexpectedly after assignment. Why is this and how can I prevent it? — Thomas Weller
– Thomas Weller, Commented Mar 4, 2022 at 12:00

Community · Accepted Answer · 2017-05-23 12:06:56Z

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, 2017 at 12:06

CommunityBot

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answered Sep 29, 2016 at 17:36

Moses Koledoye

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