How does this chain assignment work?

Question

Consider the following code; it is a bad programming practice. I am wondering why the resulting list A is [1, 1, 3] rather than [1, 2, 1]

Answer 1

A.count(3) yields 1.

t = 1 is executed first. Now t is 1.

A[t] = 1 is executed. (A[1] = 1)

---

>>> class Demo:
...     def __setitem__(self, idx, value):
...         print 'Set index', idx
... 
>>> d = Demo()
>>> d[1] = d[2] = 2
Set index 1
Set index 2