Python Function Returning List Objects Out of Order

Question

I'm relatively new to the Python language. I'm aware of most of the basic functionality & theory, all the different classes of object and their behavior/properties etc.

Anyway, I was writing basic functions to explore different concepts in practice and get to know the language more intuitively. One in particular has left me vexed! Can anyone share any insight into why this result is not as expected?

Here is the code I ran:

test_list = [2, 4, 6]
def test_func(k):
    global x, y, z
    for n in k:
        k[k.index(n)] = n * 2
    x, y, z = k
    return k
test_func(test_list)
print(test_list)
print(x)
print(y)
print(z)

I would have expected the result to be:

[4, 8, 12]
4
8
12

However, the actual result is as follows:

[8, 4, 12]
8
4
12

It seems that the first two items of the list have been swapped.

I can't see what could be causing this? If anyone can see what's happening here, please share the insight!

Thanks, Oscar South

Answer 1

In the first iteration the first element is changed to 4. In the second iteration the index of 4 is 0 not 1 as you expect. Try this:

test_list = [2, 4, 6]
def test_func(k):
    global x, y, z
    l = list()
    for n in k:
        print(n, k.index(n))
        l.append(n * 2)
    x, y, z = l
    return l
test_func(test_list)
print(test_list)
print(x)
print(y)
print(z)