The answer is A. The program prints:
9 13
Why?
Python works out everything on the right first, using the old values: b is 9 and a + b is 13. Then it puts them into a and b at the same time.
Trace table
A trace table follows the program step by step. Here’s this one:
| a | b | |
|---|---|---|
| before | 4 | 9 |
| after | 9 | 13 |
The code again
a = 4
b = 9
a, b = b, a + b
print(a, b)Try it: change one number in the program and predict the new output before you run it.