Why my lists change their value in Python?

Question

I'm a newbie with Python, and I really don't understand why occurs this:

vector1 = [1,0,0,1]
left  = vector1
print(str(left))

right = vector1
right[0] = 2

print(str(left))
print(str(right))

The output is:

[1, 0, 0, 1]
[2, 0, 0, 1]
[2, 0, 0, 1]

What I really want is:

[1, 0, 0, 1]
[1, 0, 0, 1]
[2, 0, 0, 1]

Why 'left' vector is updated if I fixed it before with the initial 'vector1' value?

I'm sure that this is a newbie question, but I'm used to code in MatLab and I'm getting confused...

Thanks in advance!

After `left = vector1`, you have two variables referring to the same list. If you want two lists, you need to copy the list. E.g. `left = list(vector1)`, `right = list(vector1)` . — khelwood, Feb 17 '17 at 10:01
Check out the execution of your code in this [link](http://www.pythontutor.com/visualize.html#code=vector1%20%3D%20%5B1,0,0,1%5D%0Aleft%20%20%3D%20vector1%0Aprint%28str%28left%29%29%0A%0Aright%20%3D%20vector1%5B%3A%5D%0Aright%5B0%5D%20%3D%202%0A%0Aprint%28str%28left%29%29%0Aprint%28str%28right%29%29&cumulative=false&curInstr=7&heapPrimitives=false&mode=display&origin=opt-frontend.js&py=3&rawInputLstJSON=%5B%5D&textReferences=false). Then try to edit the code to inspect the change. Use `left = vector1[:]` as @Darth suggests. — SSSINISTER, Feb 17 '17 at 10:06

Mithilesh Gupta · Accepted Answer · 2017-02-17T10:02:43.513

1

when you say

left  = vector1

Python does not copy the contents of list but the new list left now refers to vector1

So when you change the new list, the old list is also changed.

edited Feb 17 '17 at 10:02

answered Feb 17 '17 at 10:00

Mithilesh Gupta

And how can I copy the contents of the list into a new variable? – Víctor Martínez Feb 17 '17 at 10:01
`left = vector1[:]` will do – Darth Kotik Feb 17 '17 at 10:01
@VíctorMartínez: see the duplicate. – Martijn Pieters Feb 17 '17 at 10:01
To add to this answer, if you want to copy the contents try `left = copy.copy(vector1)` – Vlad B Feb 17 '17 at 10:01
2

This is **not** a shallow copy. It is a **reference**. A shallow copy would be a new list containing the same references as the old list. – Martijn Pieters Feb 17 '17 at 10:01
Yes , thanks for correcting :) – Mithilesh Gupta Feb 17 '17 at 10:03

1 Answers1