“num - 1” vs “num -= 1”

Question

In line 4 why do we have to add \"=\" after \"-\" ?

num = 5
if num > 2:
    print(num)
    num -= 1
print(num)

Answer 1

The = is needed to assign the result of the subtraction back to num.

The following:

num -= 1

subtracts one from num and assigns the result back to num.

On the other hand, the following:

num - 1

subtracts one from num and discards the result.