Python - append a list to the same list

Question

Hi,

I don't understand the output of this code:

a = [1,2]
a.append(a)
print (a)
print (a[2][2][2])
print (a != a[2][2][2])

Output:
[1,2, [...]]
[1,2, [...]]
False

1) why a is [1,2,[...]] and not [1,2,1,2]?
What does [...] mean?

2) why does a[2][2][2] result in [1,2,[...]]?

3) why is a the same as a[2][2][2]?

Please help.

Answer 1

By appending a list to itself, you create a self-referencing list. This is denoted by the ellipsis ([...]). It's like a recursive function (without a break condition).

list1 = [1, 2, 3]
list1.append(list1)

print(list1) # [1, 2, 3, [...]]

When you "zoom in" on the self-referencial fourth list item, you get the same list again:

print(list1[3]) # [1, 2, 3, [...]]

And this list slice's fourth element is the same as the original list again:

print(list1[3][3]) # [1, 2, 3, [...]]

You could do this forever and would always get the same result. So this is a lot like one of these fractal images.

In your example, it's [1,2,[...]] and not [1,2,1,2] because you append a reference to itself to the same list again.

Instead of saying

a = [1,2]
a.append([1, 2]),

you say

a = [1,2]
a.append(a)

which leads to the above-mentioned "recursive" effect. Another way to actually append [1, 2] to the list would be

a = [1,2]
a.append(a[:])

print(a) # [1, 2, [1, 2]]

Answer 2

In general, appending a list to another list means that you have a list item as one of your elements of your list. For example:

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

In your example, a is appended to itself, so now a is an element within itself. So when you have

a = [1,2]
a.append(a)

you've basically made a[2] the same as a. So if you were to write the whole thing out, you would get

a = [1,2,[1,2,[1,2,[1,2,[...]]]]]

That's what [...] means - you have created a bottomless rabbit hole. So a[2] == a, a[2][2] == a, a[2][2][2] == a and so on.

Answer 3

To add the elements of a list to itself you can also use extend() instead of append:

a = [1,2]
a.extend(a)
print (a)

# putput:
[1, 2, 1, 2]

Answer 4

Hi there. I don't really understand the crazy world of append, but if you don't want a multidimensional list, then you shouldn't use append, because it pushes only 1 value at the end of it. You might want to use the + (or *) operator. Example:
a = [1, 2]
a = a + a
print(a) # [1, 2, 1, 2]