I have a list x which contains another list object as below:
x = [ [1, 0] ]
Now, I am performing the concatenation operation on the list x
a = x + x
a[0][0] = 100
print(a)
Output:
[[100, 0], [100, 0]]
Here, I have not updated value of a[1][0] but it is still modified. What is causing this?
Use copy.deepcopy()
from copy import deepcopy
x = [[1, 0]]
a = deepcopy(x) + deepcopy(x)
a[0][0] = 100
Example
print(a)
Output
[[100, 0], [1, 0]]
because a after concatenation is [x,x]
a[0] is accessing x which is a variable.
and a[0][0] is implying to change x[0], thus
changing overall x, achieving [x[0],x[0]].
Try to initialize a as [[1,0],[1,0]] and run the
same command, you'll see that it'll have your desired
output or x + x.copy()
[edit]
this code:
x = [5]
y = x.copy()
x[0] += 1
print(x)
print(y)
outputs:
[6]
[5]
Another way to do this to create a copy is:
x = [ [1, 0] ]
a = [x[:],x[:]]
a[0][0] = 100
print(a)
You can create a new copy to the list instead of reference with x[:].
Output:
[[100], [[1, 0]]]
This is because both elements of a are pointing to the same element x. Try this very simple example:
x = [1,2]
y = x
y[0] = 10
print(x)
Surprisingly, the outcome is [10,2]. You can avoid this behavior using copy, which breaks this relationship and creates a new object.
a=x+x
a[0]=a[0].copy()
a[0][0]=100
Would do the job.