Why does six disappear when swapping the list of the odd number of variables in the below code?
In [1]: A = [3, 8, 9, 7, 6]
In [2]: A[:2], A[2:] = A[2:], A[:2]
In [3]: A
Out[3]: [9, 7, 3, 8]
In [4]: B = [3, 8, 9, 7, 6]
In [5]: B[2:] + B[:2]
Out[5]: [9, 7, 6, 3, 8]
In [6]: C = [3, 8, 9, 7]
In [7]: C[:2], C[2:] = C[2:], C[:2]
In [8]: C
Out[8]: [9, 7, 3, 8]
A[:2], A[2:] = A[2:], A[:2]
effectively resolves to:
# A = [3, 8, 9, 7, 6]
tmp1 = A[2:] # tmp1 = [9, 7, 6]
tmp2 = A[:2] # tmp2 = [3, 8]
A[:2] = tmp1 # A = [9, 7, 6, 9, 7, 6]
A[2:] = tmp2 # A = [9, 7, 3, 8]
List slicing is weird, and a larger slice can replace all the elements in a smaller slice, thus expanding the list. Similarly, a smaller slice can replace a larger slice, thus removing elements that used to be there.
That's what's happening here - the evaluation order, in which A[:2] gets fully evaluated before A[2:], means that the assignment to A[2:] ends up overwriting one of the values inserted by A[:2].
For a situation like this, you're better off using concatenation than multiple assignment, like you do with list B in your example.