recursion - Confusing [...] List in Python: What is it?

Question

So I was writing up a simple binary tree in Python and came across [...]

I don't believe this to be related to the Ellipsis object, more it seems to have something to do with an infinity loop (due to Python's shallow copy?). The source of this infinity loop and why it doesn't get expanded while expanding when accessed is something I'm completely lost to, however

>>> a
[[[[[], [], 8, 3], [[], [], 3, 2], 6, 3], [], 1, 4], [[], [], -4, 2], 0, 0]
>>> Keys(a)#With a+b
[0, 1, 6, 8, 3, -4]
>>> Keys(a)#With [a,b]
[8, [...], [...], 3, [...], [...], 6, [...], [...], 1, [...], [...], -4, [...], [...], 0, [...], [...]]
>>> Keys(a)[1]#??
[8, [...], [...], 3, [...], [...], 6, [...], [...], 1, [...], [...], -4, [...], [...], 0, [...], [...], 8, [...], [...], 3, [...], [...], 6, [...], [...], 1, [...], [...], -4, [...], [...], 0, [...], [...]]

Version using a+b

def Keys(x,y=[]):
    if len(x):y+=[x[2]]+Keys(x[0],y)+Keys(x[1],y)#Though it seems I was using y=y[:]+, this actually outputs an ugly mess
    return y

version using [a,b]

def Keys(x,y=[]):
    if len(x):y+=[x[2],Keys(x[0],y),Keys(x[1],y)]
    return y

So what exactly is [...]?

Answer 1

It can also appear if you have a circular structure with a list pointing to itself. Like this:

>>> a = [1,2]
>>> a.append(a)
>>> a
[1, 2, [...]]
>>> 

Since python can't print out the structure (it would be an infinite loop) it uses the ellipsis to show that there is recursion in the structure.

---

I'm not quite sure if the question was what what going on or how to fix it, but I'll try to correct the functions above.

In both of them, you first make two recursive calls, which add data to the list y, and then AGAIN append the returned data to y. This means the same data will be present several times in the result.

Either just collect all the data without adding to any y, with something like

return [x[2]]+keys(x[0])+keys(x[1])

or just do the appending in the calls, with something like

y += [x[2]]
keys(x[0], y) #Add left children to y...
keys(x[1], y) #Add right children to y...
return y

(Of course, both these snippets need handling for empty lists etc)

@Abgan also noted that you really don't want y=[] in the initializer.