Python: “a.append(a)” evaluates to a strange infinite recursive list??

Question

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

Here print(a) shows [1, 2, [...]].

So “a” is a list with an embedded list “a” with an embedded list “a” ...and so on? A pointer that references to itself?

How does it work exactly?

Answer 1

a.append(a) will create some kind of loop where the third element will be the list itself...

Basically it is

a = [1,2, a]

Where a again is equal to

a = [1,2,a]
( a = [1,2,[1,2,a]] if we expand it once)

So it can be represented as

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

So it is infinite as the a[2] = a
everytime

and the

print(a == a[2][2][2])
is true because

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

I hope you understand

Answer 2

If you imagine a referencing as an instruction to look up a certain data item in memory, you could rephrase a list lookup as:

'Please go to the address stored in slot i and give me what's there.'

So if the address of the list itself is stored in there, it will look up itself.

And if you write some code with a loop around it, trying to get to the bottom of it, it is possible to get a recursion error.

https://code.sololearn.com/c8FJrflBhwp6/?ref=app