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If I run the following code

data = [[1,2],[3,4],[5,6]]



for x in data:

    print(x[0])



for x[0] in data:

    print(x)

I get the following output

1

3

5

[[1, 2], 6]

[[3, 4], 6]

[[...], 6]

I end up with a list containing [[...], 6], but what is this [...] list?

It doesn't behave normally, because calling y = [[...], 6] and then the following statements show [...] to be 0

>>> print(y)

[[Ellipsis], 6]



>>> print(y[0])

[0]

However when I run the code at the top, and type the following statements the results don't make sense:

>>> print(x)

[[...], 6]



>>> print(x[0])

[[...], 6]



>>> print(x[0][0])

[[...], 6]



>>> print(x[0][0][0])

[[...], 6]

and yet somehow both of these result in 6

>>> print(x[1])

6



>>> print(x[0][1])

6

To review the question: How is this possible, and what does [...] represent, and how can the for loop at the top create such a list?

Let's give your sublists names:

a = [1, 2]

b = [3, 4]

c = [5, 6]

data = [a, b, c]

Your first loop binds a, b and c successively to x. When the loop terminates, you have effectively set x = c.

The second loop now binds a, b and c successively to x[0]. This is fine for a and b, but for c you are effectively doing c[0] = c, creating a circular reference. Since list is able to catch that, it won't try to print [[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[...

that's because you're using x[0] as your loop variable (which is bad practice) which exists as a list and not a new name like you're supposed to when iterating with for

for x[0] in data:

    print(x)

and x is in data so there's a cyclic reference (hence the ellipsis representation to avoid infinite recursion when printing the same data over and over)

More in detail:

The ellipsis happens on the last element because of the previous loop that binds x on the last element of data ([5,6]).

So the second loop assigns [5,6] to x[0] but it's also x. On way to get rid of this is to create a copy of x just before the second loop: x = x[:]