This is just a theoretical question and doesn't have any use case I believe.
I was playing with lists self reference and I noticed that:
>>> l = []
>>> l.append(l)
>>> l.append(l)
>>> l
[[...], [...]]
>>> l+l
[[[...], [...]], [[...], [...]], [[...], [...]], [[...], [...]]]
And ok the output seems fine? Then I tried:
>>> l = []
>>> l.append(l)
>>> l.append(l)
>>> l
[[...], [...]]
>>> l.extend(l)
>>> l
[[...], [...], [...], [...]]
I thought l+l and l.extend(l) would behave the same way with the only difference that list+list creates a copy
I then checked references but they are all the same:
>>> l = []
>>> l.append(l)
>>> l.append(l)
>>> l
[[...], [...]]
>>> l+l
[[[...], [...]], [[...], [...]], [[...], [...]], [[...], [...]]]
>>> e =l+l
>>> e[0][0] is e[0][1]
True
>>> e[0][0] is e[0]
True
>>> e[0] is e[1]
True
TLDR Why
>>> l+l
[[[...], [...]], [[...], [...]], [[...], [...]], [[...], [...]]]
But
>>> l.extend(l)
>>> l
[[...], [...], [...], [...]]
The recursive __repr__ for builtin objects checks for the first recursion and prints it.
In case of .extend() you take the list of 2 lists, and turn them into a list of 4 lists:
1
1.1 1.2 1.3 1.4
[[...], [...], [...], [...]]
Since each list is itself and prints the same, the recursion ends here.
In case of the +, the output object is not the same as the other lists:
unique
1 2 3 4
1.1 1.2 2.1 2.2 3.1 3.2 4.1 4.2
[[[...], [...]], [[...], [...]], [[...], [...]], [[...], [...]]]
You have one outer list of 4, BUT, each sub list contains 2 items. The recursion only starts when you go into depth 2.
Basically, in the first one 1.1 == 1 but in the second one 1 != unique.