Python 3.8.1 bug: list not updating correctly

Question

I have the following code: https://repl.it/repls/RottenGiddyInitializationfile

from math import *


def weights(p1,p2,p3):
    n =len(p1)
    rows, cols = (n, n)
    w = [[0]*cols]*rows

    for i in range(n):
        for j in range(n):
            if i != j:
                w[i][j] += p1[i] * p1[j]

    for i in range(n):
        for j in range(n):
            if i != j:
                w[i][j] += p2[i] * p2[j]

    for i in range(n):
        for j in range(n):
            if i != j:
                w[i][j] += p3[i] * p3[j]

    print("old")
    print(w)

    new = [[0]*cols]*rows


    for i in range(n):
      for j in range(n):
        w[i][j] = w[i][j] + 1
    print("new")
    return w



six=[1,1,-1,1,1,1,1,-1,1,1,-1]
three=[1,-1,1,1,-1,1,1,1,1,-1,-1]
one=[-1,-1,1,-1,-1,1,-1,1,-1,-1,-1]

w= weights(three,six,one)
print(w)

As evident from the console, the old value for w is some matrix of ints

I update every element by 1.

However, the code increases the value of each element by 10.

Is this a bug in python, or am I doing it incorrectly?

"Is this a bug in python" No, that shouldn't be your first assumption for this kind of thing. — juanpa.arrivillaga, Mar 05 '20 at 18:41
Anyway, see the linked duplicate. The following: `[[0]*cols]*rows` creates a list with `row` number of references **to the same list**. — juanpa.arrivillaga, Mar 05 '20 at 18:42
`[[0]*cols]*rows` in this you have same object although the list rows times — Ch3steR, Mar 05 '20 at 18:42

Python 3.8.1 bug: list not updating correctly

0 Answers0