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

On line 3 you have a chained assignment

t = A[t] = A.count(3)

t = A.count(3) is evaluated first – t set to the return value of A.count(3) which in this case is 1. Then the member of A at index t(=1) is set to the return value of A.count(3), which is still 1.

Read more about chained assignments in Python here