“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

You are essentially asking the difference between

num - 1

and

num -= 1

The former is an expression that evaluates to num - 1. The latter is an assignment that assigns num - 1 to num.

So, the former does not modify num, the latter does.