I have a list
a = [-1,4,3,1]
we have
a[1] = 4
a[3] = 1
now I want to swap a[1] and a[3]
The following code work very well:
a[1], a[3] = a[3], a[1]
But If I use:
a[1], a[a[1]-1] = a[a[1]-1], a[1]
It gives me:
a = [4,1,3,1]
The answer is not correct. What happen here since we know a[1] - 1 = 3. Any thought, thanks.
This is fundamentally the same question as Multiple assignment and evaluation order in Python with a superficial wrinkle because of the convoluted indexing.
The things to bear in mind here:
The right side is evaluated completely before the left side is evaluated at all
The tuple assignments are evaluated completely from left to right
The docs state this (albeit sparely) here: https://docs.python.org/3/reference/expressions.html#evaluation-order
Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.
Here I illustrate these rules as applied to your situation:
State of a Evaluation of expression Comment
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[a[1]-1], a[1] <start>
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[4-1], a[1] a[1] resolves to 4
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[3], a[1] 4-1 is 3
[-1, 4, 3, 1] a[1], a[a[1]-1] = 1, a[1] a[3] resolves to 1
[-1, 4, 3, 1] a[1], a[a[1]-1] = 1, 4 a[1] resolves to 4
[-1, 1, 3, 1] a[a[1]-1] = 4 a[1] = 1 assigned
[-1, 1, 3, 1] a[1-1] = 4 a[1] resolves to 1
[-1, 1, 3, 1] a[0] = 4 1-1 is 0
[4, 1, 3, 1] a[0] = 4 assigned