Why does appending a list by itself create an infinite list

Question

l = [1, 2]
l.append(l)
>>>l
[1, 2, [...]] #l is an infinite list

Why does this create an infinite list instead of creating:

l = [1, 2]
l.append(l)
>>>l
[1, 2, [1, 2]]

Answer 1

When you do:

l.append(l)

a reference to list l is appended to list l:

>>> l = [1, 2]
>>> l.append(l)
>>> l is l[2]
True
>>>

In other words, you put the list inside itself. This creates an infinite reference cycle which is represented by [...].

---

To do what you want, you need to append a copy of list l:

>>> l = [1, 2]
>>> l.append(l[:])  # Could also do 'l.append(list(l))' or 'l.append(l.copy())'
>>> l
[1, 2, [1, 2]]
>>> l is l[2]
False
>>>

Answer 2

Easy, because each object will have a reference to itself in the third element. To achieve [1, 2, [1, 2]] then use a copy of the list.

l.append(l[:])