The variable changes for no reason

I'm using Python to solve problem 27#, bubble sort. My variable original_data changes despite seemingly not being modified by the code. Here's how it looks like:

with open("data.txt", "r") as f:
    data = f.read().split()                       #take input data from the file

length = int(data.pop(0))                         

print("input data is: ",data)
print()
swaps_count = 0
finished = False

while finished == False:
    original_data = data                #make a copy of input data for later comparison
    print("original data is: ",original_data)
    result = ""
    for n in range(length):
        a,b = data[:2]
        if len(data) == 2:
            result += min(data) + " "
            result += max(data) + " "
            break
        elif int(a) < int(b):
            result += a + " "
            data = data[1:]
        else:
            swaps_count += 1
            result += b + " "
            data[1] = a
            data = data[1:]
    result = result.split()
    print("the result is: ",result)

    if result == original_data:              #if sorting is complete
        finished = True                      #break out of while loop
        print("finished!")
        break
    else:                                    #if sorting is not finished
        print("old data is: ",original_data) 
        data = result                        #start over
        print("new data is: ",data)

both data and original_data are the same at the start: ['3', '1', '4', '1', '5', '9', '2', '6']. When I run the code and compare the result of the sorting loop with original_data to determine if the sorting is finished, original_data suddenly looks different: ['3', '3', '4', '1', '5', '9', '2', '6'], and I have no idea why.

Edit: I edited the code a bit so it doesn't run forever, but the original problem with a variable changing for no reason still exists.

Be warned, the while loop doesn't work properly because of the problem, it will run forever.

I fixed the original formatting but I can't delete this comment.

You must be right. I tested it with this:

a = [1,2,3]
b = a
b.append(4)
print(a)

and appending b changes a. I thought it would be a copy and not reference to previous variable. I had that topic in my udemy course but it was long time ago. I will use that info to fix it.

Thx

I think I prefer .copy(), as then you can easily see what it does, when [:] looks obscure.