Circular Reference with python lists
That's just Python telling you that you have a circular reference; it's smart enough not to enter an infinite loop trying to print it out.
It's output by the method responsible for generating the representation of the structure. It represents a recursive structure, elided since it can be nested infinitely.
iPython will do this:
[<Recursion on list with id=38505216>]
It's the same thing; the interpreter telling you that you have a recursive data structure.