“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

Because num - 1 does nothing, but num -= 1 changes the value of num by minus one.

Answer 2

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.

Answer 3

It's a shorter version of writing:

num = num - 1