This is indeed confusing. The change is made in place, that's why A[0] becomes [2,2]. To elaborate the behavior, here is how A looks like as stored in column major:
1 3 5 2 4 6
"A[0][::-1]" has two numbers (2, 1) and their indices are 3 and 0. "A[0]" has two elements (1, 2) and their indices are 0 and 3. What "A[0]=A[0][::-1]" does is replacing the 1 (index 0) with 2 (index 3), which changes A to following:
2 3 5 2 4 6
And next step is replacing the 2 (index 3) with 1 (index 0), however, 1 is already replaced by 1 in previous step, so A doesn't change after this:
2 3 5 2 4 6
When printed out, it is ([2,2],[3,4],[5,6]).
