l=[1,2,3]
l.append(l)

In [10]: l=[1,2,3]
    ...: l.append(l)

In [11]: l is l[3]
Out[11]: True

In [12]: id(l) == id(l[3])
Out[12]: True

In [14]: l.append(l[:])

In [15]: l
Out[15]: [1, 2, 3, [1, 2, 3]]

In [17]: l.extend(l)

In [18]: l
Out[18]: [1, 2, 3, 1, 2, 3]

In [19]: l=[1,2,3]

In [20]: l[:] is l
Out[20]: False

In [21]: id(l[:])
Out[21]: 179393864

In [22]: id(l)
Out[22]: 197356744