“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

num - 1: produce the result of subtracting one from num; num is not changed

num -= 1: subtract one from num and store that result (equivalent to num = num - 1 when num is a number)

Note that you can use num - 1 as an expression since it produces a result, e.g. foo = num - 1, or print(num - 1), but you cannot use num -= 1 as an expression in Python.