python list strange behavior

Question

import numpy as np

x = np.array([1,2,3])
xlist = []
Xlist = []
for i in range(1):
    xlist.append(x**(i+1))
    Xlist.append(xlist)
xlist.append(x**2)
Xlist.append(xlist)
Xlist

However, the output is

[[array([1, 2, 3]), array([1, 4, 9])], [array([1, 2, 3]), array([1, 4, 9])]]

However, I don't know why the result isn't the expression below, and I want the result below, how to achieve that?

[[array([1, 2, 3])], [array([1, 2, 3]), array([1, 4, 9])]]

@QuentinPradet this is so surprising! How can you know that ?! — ArtificiallyIntelligence, Feb 19 '16 at 07:34
@Nishant I hadn't used extend before. I thought "+" is only applicable to strings. — ArtificiallyIntelligence, Feb 19 '16 at 07:35
You are not creating copies, you are just appending the same list object over and over. See the dupe on how to properly copy. — Martijn Pieters, Feb 19 '16 at 07:35
@PabPeter Because of the "Xlist" at the end which I supposed you used to display Xlist in your notebook. This was the most likely explanation for LPK's question. — Quentin Pradet, Feb 19 '16 at 07:36

score 0 · Answer 1 · answered Feb 19 '16 at 07:33

you need to create a copy of the list first then append the sqaured array.

new_list = xlist+x**(i+1)
xlist.append(new_list)

your problem is occuring because np.array when appended are passed as reference so if you change it outside the list, the value will change in the list as well.

python list strange behavior

1 Answers1