The indentation is broken, so one possible way to interpret it is: def is_prime(n: int): result = True for k in range(2, int(n ** 0.5) + 1):

The indentation is broken, so one possible way to interpret it is: def is_prime(n: int): result = True for k in range(2, int(n ** 0.5) + 1): if n % k == 0: result = False break return result Which is just a more complicated (n % 2 != 0). Obviously the break should be in the if block.