What do ellipsis […] mean in a list?

Question

I was playing around in python. I used the following code in IDLE:

p  = [1, 2]
p[1:1] = [p]
print p

The output was:

[1, [.         


        

Answer 1

We do this all the time in object-oriented programming. If any two objects refer to each other, directly or indirectly, they are both infinitely recursive structures (or both part of the same infinitely recursive structure, depending on how you look at it). That's why you don't see this much in something as primitive as a list -- because we're usually better off describing the concept as interconnected "objects" than an "infinite list".

You can also get ... with an infinitely recursive dictionary. Let's say you want a dictionary of the corners of a triangle, where each value is a dictionary of the other corners connected to that corner. You could set it up like this:

a = {}
b = {}
c = {}
triangle = {"a": a, "b": b, "c": c}
a["b"] = b
a["c"] = c
b["a"] = a
b["c"] = c
c["a"] = a
c["b"] = b

Now if you print triangle (or a or b or c for that matter), you'll see it's full of {...} because any two corners are referring to back to each other.