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

[bowlofred]

First, list and iter are names of classes in python. You shouldn't use them as variable names or you hide the ability to use the class namespaces.

You don't have anything named oldmatrix in your code, but I can guess what you've done is assumed that assigning a list copies the data rather than just a reference to it. (Please put your code in "python" tags so that the indentation is preserved).

I've made some other code to show how this can affect you and one way to avoid it.

baselist = [0, 0, 0, 0]
working_list = baselist # copies the object, not the elements inside

working_list[2] = "XX"
print(f"{working_list=}")
print(f"{baselist=}")
print()

# Note "baselist" has changed without explicitly doing so because it is the same object
# as working_list.

baselist = [0, 0, 0, 0]
working_list = baselist.copy()  # This is a copy and the two objects are different

working_list[2] = "XX"  # doesn't change baselist now.
print(f"{working_list=}")
print(f"{baselist=}")

Output:
working_list=[0, 0, 'XX', 0]
baselist=[0, 0, 'XX', 0]

working_list=[0, 0, 'XX', 0]
baselist=[0, 0, 0, 0]