Trying to find the next prime number

[deanhystad]

Once you enter the loop it does not care if you change next_num, so your outer loop only tests if num + 1 is prime, and it doesn't do that correctly.

This is not a good test for prime:

        for i in range(2, next_num):
            if (next_num % i) == 0:
                next_num += 1
            else:
                break
                print(f"The next prime number is {next_num}.")

Let's say next_num = 9. You test if 9 is evenly divisible by 2. 9 % 2 == 1. It is not divisible by 2, so according to your program it is prime.

Finding only one factor is enough to determine a number is not prime. You need to test all possible factors before you can say a number is prime.