Why is a variable automatically modified even though I am not modifying it?

Question

I simplified my code to get the error I required.

one = ['x']
two = ['y']
final_data = []
for r in range(1, 3):
    print(final_data, r)
    if r == 2:
        one[0] = two[0]
        print(one)
    final_data.append(one)
    print(final_data)
print(final_data)

The final data in 2nd loop is not modified but the end result is coming [['y'], ['y']] even though I expect it to come as [['x'], ['y']]

Answer 1

In this line one[0] = two[0] you change the value in list one and in the end when you print final_data you get [['y'], ['y']] because in list one you have 'y'.

You need for each iteration to create a copy of one. To copy a list you can use [:]. Then after you do what you want with one, copy old_one to one.

...
old_one = one[:]
...
one = old_one[:]

Try this:

one = ['x']
two = ['y']
final_data = []
for r in range(1, 3):
    print(final_data, r)
    old_one = one[:]
    if r == 2:
        one[0] = two[0]
        print(one)
    final_data.append(one)
    one = old_one[:]
    print(final_data)
print(final_data)