Python List append

Question

Can you explain the behind the scenes of the output of this code ?

a = [1,2]
b = [1,2]
a.append (b)
b.append (a)
print(a)
print(b)
https://code.sololearn.com/cCFgkfxZwrUz/?ref=app

Answer 1

Both append statements add a third element to the list, which is a reference to the other list. Therefore it creates an endless circular reference between these two lists.
Python detects this infinite loop and expresses it with the [...] or Ellipsis symbol.
Check out this thread too:
https://www.sololearn.com/discuss/1874398/?ref=app

Answer 2

Thank you Tibor Santa