how can a variable change if I haven't changed it?

[niminim]

You need to do a deep copy

import copy
a = [[1, 1], [2, 2]]

b = a.copy()
print('Copy')
for x, y in zip(a, b):
    print(x, y, id(x), id(y))


print('\nDeep Copy')
b = copy.deepcopy(a)
for x, y in zip(a, b):
    print(x, y, id(x), id(y))

Output:
Copy
[1, 1] [1, 1] 2858215123584 2858215123584
[2, 2] [2, 2] 2858215123072 2858215123072

Deep Copy
[1, 1] [1, 1] 2858215123584 2858215122240
[2, 2] [2, 2] 2858215123072 2858215123520

When you copy a list of lists the copy is a new list that contains lists from the original. To create copies of the contents you should use deepcopy from the copy library. Notice that the object ID's match when doing b = a.copy(), but when doing b = copy.deepcopy(a), new lists are created for the copy.

Thanks pal, you've solved my problem and taught me a lesson.