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?

CodePudding user response：

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]]

CodePudding user response：

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.

CodePudding user response：

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]]]

CodePudding user response：

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]