Why does my numpy matrix get updated before get assigned?

Question

This is my code

import numpy as np
v = np.zeros(4)
backup = np.zeros(4)

for i in range(3):
   backup = v
   v[0] = 1
   print(backup)

My output is:

[1. 0. 0. 0.]
[1. 0. 0. 0.]
[1. 0. 0. 0.]

But I expected:

[0. 0. 0. 0.]
[1. 0. 0. 0.]
[1. 0. 0. 0.]

Why does backup matrix get updated before assignment?

Answer 1

You are not really doing back up: you are just making another reference.

Making a copy solves it:

import numpy as np
v = np.zeros(4)
backup = np.zeros(4)

for i in range(3):
    backup = v.copy()
    v[0] = 1
    print(backup)

[0. 0. 0. 0.]
[1. 0. 0. 0.]
[1. 0. 0. 0.]

Answer 2

I think backup = v is not making a copy its is just assigning the values.