Confusing […] List in Python: What is it?

>>> 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, [...], [...]]

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

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

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

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

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

>>> a = [0, 1]
>>> a[0] = a
>>> a
[[...], 1]

>>> a = [0, 1]
>>> a[0] = a
>>> a
[[...], 1]

>>> a = [0, 1]
>>> a[0] = a
>>> a
[[...], 1]

>>> a = [0, 1]
>>> a[0] = a
>>> a
[[...], 1]

>>> a = [0, 1]
>>> a[0] = a
>>> a
[[...], 1]

>>> a = [0]
>>> a.append(a)
>>> print a
[0, [...]]

[0, [...]]

[0, [0, [...]]]

[0, [0, [0, [...]]]]
[0, [0, [0, [0, [...]]]]]
[0, [0, [0, [0, [0, [...]]]]]]
...

print

y = y[:]

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

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

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

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

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