Self-containing lists...?!?
Ryan Westlund
・1 min read
So today I was reading the Python Design and History FAQ and I learned about a really weird feature: Lists can contain themselves!
>>> a = [1, 2]
>>> a.append(a)
>>> a
[1, 2, [...]]
>>> a[2][2][2]
[1, 2, [...]]
>>> a[2][2][2][0]
1
>>> a[0] = a
>>> a
[[...], 2, [...]]
>>> a = [a,a,a]
>>> a
[[[...], 2, [...]], [[...], 2, [...]], [[...], 2, [...]]]
Well... mind blown...
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Discussion
Lol, it kind of is my first language - or was. But I've known it for almost 7 years and know at least 5 other general-purpose languages (Go, Javascript, Haskell, Rust, C).
How is this "commonly" used?
This is something I sort of knew but haven't given much thought. I wonder if the element access is constrained by the max recursion-depth? Btw, this is the reason python has a garbage collector. Reference counting breakers down with circular dependencies like this.
I looked into this and wrote up a post on my findings:
TIL: Python Recursion Limit When Accessing Elements in Recursive List
Fredrik Bengtsson ・ Aug 5 ・ 2 min read
Wow
Now print our the contents of a
It has same result as returning a in python console. Including for-loop. Flattening doesn't help too.
Why would you think it's satire?
How is it different from circular references?
Hmm I guess I just never rthought of it as an extension of objects pointing to each other
It is just contain reference to itself.
It is called circular list.