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

Answer 1

Ok, so in points:

1. You're creating infinite data structure:

   def Keys(x,y=[])

   will use the same 'y' in each call. This just isn't correct.

2. The print statement, however, is clever enough not to print an infinite data, but to mark self-reference with a [...] (known as Ellipsis)

3. The Python will allow you to address such structure correctly, so you can write

   a.keys()[1][1][1]

   and so on. Why shouldn't you?
4. The y = y[:] statement simply copies the list y. Can be done more soundly with y = list(y)

Try using the following code:

def Keys(x,y=None):
    if y is None:
        y = []
    if len(x):
        y += [x[2], Keys(x[0],y), Keys(x[1],y)]
    return y

But still I guess that it can bite you. You're still using the same variable y (I mean the same object) in three places in one expression:

 y += [x[2], Keys(x[0], y), Keys(x[1], y)] 

Is that what you really want to achieve? Or maybe you should try:

def mKeys(x,y=None):
    if y is None:
        y = []
    if len(x):
       z = [x[2], mKeys(x[0], y), mKeys(x[1],y)]
       return z
   return []